adhesion theories in wood adhesive bondingdownload.xuebalib.com/xuebalib.com.12740.pdf · adhesion...

*Corresponding author: [email protected]*Corresponding author: [email protected]

K.L. Mittal (ed.) Progress in Adhesion and Adhesives, (125–168) © 2015 Scrivener Publishing LLC

5

Adhesion Theories in Wood Adhesive BondingDouglas J. Gardner*, Melanie Blumentritt, Lu Wang and Nadir Yildirim

University of Maine, Advanced Structures and Composites Center, Orono, Maine, USA

AbstractInvestigating the theories or mechanisms responsible for wood adhesive bonding has been an important aspect of wood science and technology research over the past century. Understanding the nature of adhesion in wood and wood-based composites is of importance because of the fact that wood is adhesively bonded in over 80 percent of its applications. For wood bonding, studying adhesion theories requires an understanding of wood mate-rial characteristics, surface science, polymer characteristics, and the interactions between polymers and surfaces. The state-of-the-art categorizes adhesion theories or mechanisms into seven models or areas. These are: mechanical interlocking; electronic or electrostatic theory; adsorption (thermodynamic) or wetting theory; diffusion theory; chemical (cova-lent) bonding theory; acid-base theory; and theory of weak boundary layers. The goal of this paper is to provide a concise, critical, state-of-the-art review on adhesion theories in wood adhesive bonding with an emphasis on factors influencing bond creation in wood-based material applications. Over 200 papers were reviewed and information is presented with recommendations for future studies on wood adhesion.

Keywords: Adhesion, theories, covalent bonding, diffusion, mechanical interlocking, electrostatic, weak boundary layer, wetting, acid-base, wood

5.1 Introduction

It has been recognized over the past several decades that understanding the nature of adhe-sion in wood and wood-based composites is of great importance because of the fact that wood is adhesively bonded in over 80 percent of its applications [1]. There is a plethora of wood and wood-based composite products that rely on adhesives in their manufacturing

126 Progress in Adhesion and Adhesives

processes [2]. Wood composite products include: glulam beams, oriented strand board, medium density fiberboard, particleboard, and paper just to mention a few [3]. Studying the theories or mechanisms responsible for wood adhesive bonding has been an important aspect of wood science and technology research over the past century. It is anticipated that improvements in the understanding of wood adhesion mechanisms have the potential to result in better adhesive systems and more efficient and effective processing methods for the wide array of wood and wood-based composite materials.

For wood bonding, studying adhesion theories requires an understanding of wood mate-rial characteristics, surface science, polymer characteristics, and the interactions between polymers and surfaces. At present no practical unifying theory describing all adhesive bonds exists, although a unified adhesion theory has been proposed [4]. The state-of-the-art categorizes adhesion theories or mechanisms into seven models or areas [5–7]. These are:

1. Mechanical interlocking theory2. Electronic or electrostatic theory3. Adsorption (thermodynamic) or wetting theory4. Diffusion theory5. Chemical (covalent) bonding theory6. Acid-base theory7. Theory of weak boundary layers

It should be noted that these theories are not self-excluding, and several may be occur-ring at the same time in a given adhesive bond depending on the particular circumstance. Recent applications of adhesion theories to describing the nature of wood adhesive bonding have focused effort on the durability of wood adhesive bonds [8–11].

5.1.1 Wood Material Properties Relevant to Adhesion

Wood can be classified into two broad groupings, i.e., softwoods and hardwoods [12]. Softwoods are from gymnosperms or those trees with needle-like leaves, generally ever-green, and bearing seeds in a woody cone. Hardwoods are from angiosperms or those trees with broad, deciduous leaves and bearing seeds in a fleshy fruit. Wood can be classified as an orthotropic, heterogeneous, cellular solid. The structure and properties of wood differ in three mutually-orthogonal planes: transverse (cross section), radial, and tangential. In the tree, wood can also be segregated into heartwood and sapwood. Sapwood, the periphery of the tree stem, is generally light in color, transports water, and stores food in the living tree. Heartwood, the central portion of the stem, is often darker in color, and contains extractives (fatty acids, waxes, etc.). The cellular structure of wood also varies between softwoods and hardwoods. Longitudinal tracheids account for over 90 percent of softwood volume, and the empty cell lumens suggest a honeycomb structure. Hardwoods contain fibers (analo-gous to softwood tracheids) that provide mechanical strength to the tree and large diameter vessel elements (“pores”) that mainly transport water and nutrients. The arrangement of cells and volumetric composition varies greatly among species. Softwood tracheids range

Adhesion Theories in Wood Adhesive Bonding 127

in size from 30 to 40 µm in diameter, and 3 to 4 mm in length. Hardwood vessel elements range from 20 to 300 µm in diameter and 0.5 to 1.5 mm in length while hardwood fibers range from 10 to 20 µm in diameter and 1 to 2 mm in length [3].

The chemical composition of wood also varies among species but the major structural organic polymers are similar in composition. Wood contains about 50 percent cellulose, 20 to 35 percent hemicellulose, and 16 to 33 percent lignin, on a dry weight basis [3]. Cellulose and hemicellulose are carbohydrate based polymers while lignin is a phenylpropane-based polymer. The non-structural components include extractives, typically 4 to 10 percent, and inorganic ash (0.2 to 0.5 percent in softwoods, 0.1 to 1.4 percent in hardwoods). Wood also contains water. Freshly cut trees may contain 30 to 200 percent or more water based on the oven dry weight of wood while “dry wood” contains 5 to 12 percent moisture content depending on the ambient interior environment, or up to 20 percent moisture content in exterior environments.

A simplified view of the cell wall structure suggests that cellulose microfibrils are embed-ded in a discontinuous lignin/hemicellulose matrix. Microfibril orientation varies in the wood cells as a function of the particular wall layer. Layers in the cell wall include the middle lamella between wood cells, the primary and three secondary wall layers: S1, S2, and S3. Several softwoods also exhibit a so-called “warty layer”. The extractives in wood are low molecular weight organic chemicals specific to particular species and they contribute to wood odor, can provide decay resistance, concentrate at the wood surface during drying, and can negatively impact adhesion and finishing.

Wood adhesion is dictated by its anatomical, chemical, mechanical, and physical prop-erties. The differences among wood species as a function of anatomy, chemistry, mechani-cal or physical properties reinforce its description as having a heterogeneous nature. Important characteristics of wood relevant to wood adhesion processes are wood’s poros-ity, anisotropy, dimensional instability, and wood surface properties. Wood as a cellular material is porous and exhibits differing levels of porosity depending on species. Being an anisotropic material, wood exhibits different physical and mechanical properties depend-ing on the orientation of the wood element. Because of its hygroscopic nature wood swells and shrinks as a function of moisture content, thus contributing to its dimensional instability.

Wood surface property issues include chemical heterogeneity, surface inactivation, weak boundary layers, and processing characteristics, i.e., machining, drying, and aging [13]. From a chemical perspective, it has long been recognized that extractives dominate wood surface properties [14, 15]. Because of the complex nature of wood structure and the myr-iad types of wood species, it is difficult to make sweeping generalizations about the surface properties of wood. It should be emphasized that although there are certain surface prop-erty behaviors that are similar among different wood species, it is usually prudent to learn about the specific species of wood type being adhesively bonded.

Approximately 50 years ago, a generalized representation of wood elements used in wood composites was promulgated that provided a practical length scale for wood com-posite elements [16]. The wood elements listed include logs, lumber, veneer, strands, chips, flakes, excelsior, particles, fiber bundles, fibers and wood flour. We have adapted this


representation and added microcrystalline cellulose and cellulose nanofibers, and a ques-tion mark is left in the table for elements not yet defined (Figure 5.1) [17, 18].

A practical look at the orders of scale for examining wood-adhesive interactions from the 1 meter scale down to the 1 nanometer scale are listed in Table 5.1. Evaluations of gross laminate adhesion failure surfaces in glulam beams are determined on the 1 m length scale whereas delamination measurements on glulam beam specimens subjected to accelerated aging regimes are made on the 10 cm length scale. Plywood lap shear specimen percent wood failure measurements are made on the centimeter length scale. Interactions between polymer droplets on individual cellulose fibers occur on the millimeter length scale, and microscopic evaluation of the wood-adhesive bondline is carried out on the 100 µm length scale. A bordered pit on a softwood tracheid is 10 µm in diameter, and the smallest resin droplets on medium density fiberboard furnish are on the order of 1 to 10 µm in diameter. Cellulose nanofibrils are on the scale of 100 nm in length and 10 to 20 nm in diameter.

The comparison of wood-adhesive interactions relative to length scale is shown in Table 5.2. Wood as a porous, cellular material has roughness on the micrometer scale but can also exhibit roughness on the millimeter scale depending on how a particular wood element to be bonded is produced. For example, production of rotary peeled veneer can produce roughness on a millimeter scale because of the creation of lathe checks. Fibers can exhibit roughness on the micro- or nanoscale depending on the method of preparation. Pores or free volume also occurs within the amorphous regions of the cell wall material on the molecular level.

Figure 5.1 Wood elements adapted from [16]. The question mark (?) denotes wood elements not yet defined.


5.1.2 Objectives

The goal of this paper is to provide a concise, critical, state-of-the-art review on adhesion theories in wood adhesive bonding with an emphasis on factors influencing bond creation in wood-based material applications.

5.2 Mechanical Interlocking and Mechanics of Adhesive-Wood Interactions

Mechanical interlocking, which was proposed by McBain and Hopkins in the early 1900’s [19] is one of the basic adhesion mechanisms that can be divided into two groups, specifi-cally: locking by friction and locking by dovetailing (Figure 5.2) [20]. In mechanically inter-locked systems, there are irregularities, pores or crevices where adhesives penetrate into and are adhered mechanically [21]. Mechanical interlocking strongly depends on the geometry

Table 5.1 Orders of scale for wood-adhesive interactions.

Scale Test specimen or material characteristic for determining wood-adhesive interactions

1 meter, 100 cm Glulam beam laminates10-1 meter, 10 cm Glulam cycle delamination specimens10-2 meter, 1 cm Plywood lap shear test specimens10-3 meter, 1 mm Polymer microdroplet on wood or cellulose fiber10-4 meter, 100 µm Microscopic evaluation of wood-adhesive bondline10-5 meter, 10 µm Diameter of bordered pit10-6 meter, 1 µm Smallest resin droplets on medium density fiberboard

furnish10-7 meter, 100 nm Scale of cellulose nanofibrils

10-8 – 10-9 meters, 1 to 10 nm Scale of wood cell wall polymers

Table 5.2 Comparison of wood-adhesive interactions relative to length scale.*

Component µm nm

Adhesion forces 0.0002–0.0003 0.2–0.3Cell wall pore diameter 0.0017–0.002 1.7–2.0PF resin molecular length 0.0015–0.005 1.5–5.0Diameter of particles that can pass through a pit 0.2 200Tracheid lumen diameter 4–25

Glueline thickness 50–250

*Adapted from [203]


of the bonding sites and the mechanical properties of the materials involved [20]. Wood adhesive bonding is a good example where mechanical interlocking is observed because of wood’s porous structure where it has open cells on the surface so that the adhesive can flow into the lumen of the cells and provide high strength [2].

In addition to geometry factors, surface roughness has a big effect on adhesion. Rougher surfaces provide better adhesion than smooth surfaces. Gent and Lin (1990) showed that a rough surface with a 60° peak angle has twice as much surface area as a flat surface [22]. Based on a model by Marra (1992), the linkage (chain link analogy) of a wood adhesive bond (Figure 5.3) can be divided into 9 links [16], where link 1 is the pure adhesive, link 2 and link 3 are the adhesive boundary layer which is no longer homogeneous because of the substrate influence while cured. Adhesion mechanisms may be mechanical interlocking, or chemical bonding which can be seen in links 4 and 5. Links 6, 7, 8 and 9 are the wood cells for this example, which are shown in Figure 5.3.

A study investigating the relationship between surface roughness and peel strength (N/m) showed that an increase in surface roughness produces an increase in the surface area (contact area), which produces higher peel strengths [23]. Wake (1982) proposed an equation defining the effects of mechanical interlocking and thermodynamic interfacial interactions for esti-mating adhesive joint strength G, as:

G = C × Mk × Ii (5.1)

where: C is a constant, Mk is a mechanical keying component, and Ii is the interfacial inter-actions component [24].

Figure 5.2 Schematic diagram of mechanical interlocking mechanisms.


High-level adhesion can be attained by improving the surface properties and mechanical keying can be enhanced by increasing the surface area [24]. Cheng and Sun (2006) studied the adhesion between a soybean protein adhesive and wood, where they investigated the effects of wood surface roughness, adhesive viscosity, and the processing pressure on adhe-sion strength [25].

On the other hand, absorption has an important role in mechanical interlocking, because absorption affects the penetration of a liquid into pores or irregularities on the adherend surface. Therefore, higher absorption produces better adhesion in mechanical interlocking systems [26]. The length scale, which changes according to type of interaction, is another fac-tor that affects adhesion. The detailed length scales for adhesion are listed in Table 5.3 [11].

Mechanical interlocking is an important factor for adhesion in wood and affects the mechanical stability of end grain adhesive joints. Follrich et al. (2007) examined grain angle effects on shear strength and showed that good mechanical interlocking is an important fac-tor for the strength of adhesive joints [27]. In another study on the thermal and mechanical properties of polypropylene (PP)-wood powder (WP) composites, it was determined that better interfacial adhesion can be obtained with better mechanical interlocking [28]. Smith

Figure 5.3 Chain link analogy for an adhesive bond in wood [16].

Table 5.3 Comparison of adhesion interactions relative to length scale.

Category of Adhesion Mechanism Type of Interaction Length Scale

Mechanical Interlocking or entanglement 0.01–1000 μm

Diffusion Interlocking or entanglement 10 nm–2 μm

Electrostatic Charge 0.1–1.0 μm

Covalent bonding Charge 0.1–0.2 nm

Acid-base interaction Charge 0.1–0.4 nm

Lifshitz-van der Waals Charge 0.5–1.0 nm


et al. (2002) used scanning electron microscopy (SEM) to investigate mechanical inter-locking of a thermoplastic adhesive and wood substrate. They showed that the extent of mechanical interlocking depends on the processing details of the adhesive joints and higher pressure and higher temperature with increased process time provides better interlocking than a short process cycle [29]. Kazayawoko et al. (1999) investigated the effects of adhe-sion on wood-polymer composites’ mechanical performance and indicated that a decrease in the quality of mechanical interlocking because of pores or crevices produced lower shear strength [30]. A study on mechanical interlocking of adhesive in the cellular structure of wood showed that mechanical interlocking is the only effective bonding mechanism in preservative-treated wood because cell walls are physically blocked and chemically modi-fied by metal complexes [31]. Backman and Lindberg (2004) investigated the interaction of Pinus sylvestris wood with poly (vinyl acetate) (PVAc) adhesive, poly(methyl methacrylate) (PMMA), and a hydrophilic acrylate, and, as a result, found that adhesion between wood and PVAc adhesive is mainly attributed to mechanical interlocking [32].

Mechanical interlocking is strongly dependent on the surface properties. While studying mechanical interlocking, the surface properties including the presence of crevices, pores, roughness and irregularities should be well understood. Optimizing the surface proper-ties, for instance, increasing the roughness, of the surface will produce stronger or better mechanical interlocking.

5.2.1 Atomic Force Microscopy (AFM) & Nanoindentation

The atomic force microscope (AFM), an instrument that allows the measurement of surface characteristics using different interaction modes, is a useful tool to accurately measure the adhesion and pull-off forces between two surfaces. AFM allows researchers to investigate the adhesion properties of materials on the micro- and nano- levels so it is a valuable tool to aid in the understanding of the interactions in wood adhesive bonding systems where wood is comprised of micro- and nano- structures [33].

Over the last decade, there has been a significant increase in adhesion and pull-off force determinations, which depend on contact mechanics, specifically: Hertz, JKR (Johnson, Kendall and Roberts) and DMT (Derjaguin, Muller and Toporov). JKR and DMT incorpo-rate the concepts developed by Hertz in 1896 [34]. The equations responsible for determi-nation of the contact radius and pull-off forces according to Hertz, JKR and DMT are given in Table 5.4 [35]. The comparison of the theories discussed above is based on interactions between a flat plane and a sphere depicted in Figure 5.4 [35]. There is no attractive force in the Hertz model, only hard wall repulsion at contact. The JKR model includes short-range adhesion which is essentially a delta function with strength WA, and thus only acts within the contact zone. The DMT curve shown represents a long-range surface force between the surface and a sphere. A volume integrated force, like the van der Waals force, can also lead to a DMT dependence, where the contact profile remains Hertzian and the attractive forces act like an additional external load. For an actual interaction force, the integral of the attrac-tive well corresponds to the work of adhesion, WA.


Surface roughness characterization and adhesion force measurements have become an important topic with the increase in nanotechnology research and improvements in devices like AFM [36]. Hertz, JKR, and DMT theories have been modified and modeled by differ-ent researchers and applied to various materials [37]. A study on determining the adhesion between two rough surfaces showed that the modified model based on JKR theory may provide a useful approach for researchers studying adhesion [37].

Another technique that is used to measure the effect of adhesives and the adhesion between adhesives and wood is nanoindentation, which provides a more accurate way to make adhesion measurements. This technique allows researchers to apply the force to the surface vertically and protect the tip sliding on the surface and creating greater adhesion forces due to friction. Jakes et al. (2008) devised an experimental method to account for structural compliance in nanoindentation measurements, where they utilized the standard Oliver–Pharr [38] nanoindentation analysis where assumptions of the model include hav-ing a structurally rigid specimen with a homogeneous structure [39]. As a result of their

Table 5.4 Equations responsible for determination of the contact radius and pull-off forces according to Hertz, JKR and DMT.

Equation Method Source

=

1 3PRa

K

Hertz [204]

ν ν

−

− −= +

12 2

1 2

1 2

1 143

KE E

Hertz [204]

( )γπ γπ γπ

= ∗ + + +

1/323 ) (6 ) (3 )

Ra P R RP R

K

JKR [35]

γπ= −32cP R

JKR [34]

( γπ

= ∗ +

1/3

2 )R

a P RK

DMT [205]

Pc = –2gπR DMT [205]where;P= Loada= Contact radiusR= Sphere radiusE1, E2= Young’s moduliK= Combined elastic modulus of tipν1, ν2= Sphere and flat-plane Poisson ratiosg = Dupré energy of adhesionPc= Pull-off force


experimental study, they found a small amount of adhesion between the tip of the indenter and wood tracheid wall during the final unloading, which is shown in Figure 5.5 [39].

Konnerth and Gindl (2006) measured elastic modulus, hardness, and creep factor of wood cell walls in the interphase region of four different adhesive bonds specifically, melamine-urea-formaldehyde (MUF), phenol-resorcinol-formaldehyde (PRF), poly(vinyl acetate) (PVAc) and polyurethane (PU). They found that MUF and PRF bondlines produced improvement in elastic modulus and hardness and reduced the creep compared to reference cell walls (unaf-fected by adhesive) whereas PVAc and PUR bondlines decreased the elastic modulus and

Figure 5.5 Typical load-depth (displacement) curve (drift subtracted) for multi-load indents performed in S2 layer of wood tracheid wall [39].

Figure 5.4 Interaction forces (per unit area) for the Hertz, JKR and DMT models, compared to an actual interaction. There is no attractive force in the Hertz model, only hard wall repulsion at contact. The JKR model includes short-range adhesion that is essentially a delta function with bond strength WA , and thus only acts within the contact zone. The DMT curve shown represents a long-range surface force. A volume integrated force, like the van der Waals force, can also lead to a DMT dependence, where the contact profile remains Hertzian and the attractive force acts like an additional external load. For an actual interaction force, the integral of the attractive well corresponds to the work of adhesion, WA [35].


hardness and increased the creep. A sample optical micrograph, showing the tested (nanoin-dented) area of a wood joint with PVAc bondline, is provided in Figure 5.6 [40].

5.3 Electrostatic Adhesion

The electrostatic theory of adhesion was first proposed by Derjaguin in 1948 [41]. In the electrostatic theory, the adherend-adherend interface is viewed as analogous to the plates of an electrical condenser across which charge transfer occurs and adhesion strength is attributed to electrostatic forces (Figure 5.7) [6]. A list of the concepts and quantities important in electrostatic adhesion are shown in Table 5.5. Coulomb’s Law describes the electrostatic interaction between electrically charged particles (Figure 5.8) as:

=

1 22e

q qF k

r (5.2)

where: F is force, ke is Coulomb’s constant, q1 and q2 are the charges and r is the distance between the charges. Capacitance C is defined as the ratio of charge Q on each conductor to the voltage V between them

=

QC

V (5.3)

Derjaguin expressed the force F(h) acting between two charges away from one another to the strength of an adhesion bond where:

F (h) = 2π Reff W(h) (5.4)where: W (h) is the interaction energy per unit area between the two planar walls and Reff the effective radius.

Figure 5.6 Optical micrograph of a wood-adhesive joint with a PVAc bondline (indicated by arrows) [40].


Table 5.5 Concepts and quantities important in electrostatic adhesion*.

Concept Definition

Electric field Generated by electrically charged particlesCoulomb’s Law Electrostatic interaction between electrically charged particles.Capacitor Consists of two conductors separated by a non-conductive region.Charge density Measure of electric charge per unit volume of space, in one, two or three

dimensions.Van der Waals force Close-range force between two molecules attributed to their dipole

momentsHamaker constant Augmentation factor for van der Waals force when many molecules are

involved, as in the case of nanoparticlesDLVO Theory Named after Derjaguin, Landau, Verwey and Overbeek. Theory explains

the aggregation of particles in aqueous dispersions quantitatively and describes the force between charged surfaces interacting through a liquid medium. It combines the effects of the van der Waals attraction and the electrostatic repulsion due to the so-called double layer of counter ions.

Zeta potential The potential difference between the dispersion medium and the station-ary layer of liquid attached to the dispersed particle

Smoluchowsky approximation

Used to calculate the zeta potentials of dispersed spherical nanoparticles

*Adapted and augmented from [43]

Figure 5.8 Interaction between electrically charged particles. F1 and F2 are the forces of interaction between two point charges (q1 and q2) and the distance (r) between them.

Figure 5.7 Schematic of the formation of an adhesion bond attributed to transfer of charge from an electropositive material to an electronegative material.


When dealing with electrostatic interactions in liquids, the Derjaguin, Landau, Verwey and Overbeek (DLVO) theory is used to describe interactions between charged surfaces where the total adhesion force FA is equal to the sum of the van der Waals force FvdW, and the Electric Double Layer force FEDL

FA = FndW + FEDL (5.5)The van der Waals force is a function of the system Hamaker constant, particle diameter,

contact radius and particle-surface separation distance. The Electric Double Layer force is a function of liquid medium dielectric constant, zeta potential, reciprocal double layer thick-ness, particle diameter, and particle-surface separation distance.

The electrostatic theory is often used to describe adhesion behavior of pow-ders to solid surfaces [41–43]. Practical applications of electrostatic adhe-sion in wood and wood-based materials include coating of furniture, sandpaper manufacture, xerography or photocopying paper, and ink jet and laser jet printing [44–46]. Typically, dry particles or powders (coatings, inks) are charged and deposited on an oppo-sitely charged or grounded substrate. A novel application of inkjet printing is in the creation of bioactive papers for cellulose-based functional materials [47]. Waterborne coatings can also be applied to wood electrostatically [48]. Electrostatic coating of wood became viable and commercially applicable in the 1950s [49, 50]. However, the scientific literature has been somewhat limited on electrostatic adhesion applied to wood over the past 60 years. It should be noted that the patent literature on electrostatic powder coating over the same time period is significant [51].

Electrostatic adhesion that occurs in the liquid phase through colloidal interactions has received much greater emphasis in the scientific literature and practical applications are plentiful in paper manufacturing [52]. Electrostatic interactions have also been the subject of study regarding cellulose films [11]. Electrostatic self-assembly in liquids is an important area in nanoscience applications [43, 53] and has been applied to paper coatings with success [54]. Electrostatic self-assembly has also been applied to wood using layer-by-layer (LbL) nanoscale coatings (Figure 5.9) and the creation of conductive paper using nanoparticles [55–58].

5.4 Wettability, Surface Energy, Thermodynamic Adhesion

Thermodynamic adhesion or wetting refers to the atomic and molecular interac-tions between adhesives and adherends. Surface tension or surface energy represent these forces and are regarded as fundamental material properties to understand adhe-sion because they are associated with adhesive bond formation [5]. Bond formation arises from the highly localized intermolecular interaction forces between materials. Therefore, good wetting is beneficial to strong adhesive bonding. Mittal pointed out that the dominant surface chemical and energetic factor influencing joint strength is interfa-cial tension between the adhesive and the adherend (gsl): the joint strength increases as gsl decreases [59]. The atomic and molecular forces involved in wetting include: (a) acid-base interactions, (b) weak hydrogen bonding, or (c) van der Waals forces (dipole-dipole


and dispersion forces) [5]. The condition necessary for spontaneous wetting is given below:

gsg ≥ gsl + glg (5.6)where: gsg, gsl and glg are respectively the interfacial free energies for solid-gas, solid-liquid and liquid-gas interfaces. If gsl is insignificant, the criterion can be simplified to:

gsg ≥ glg or gsubstrate ≥ gadhesive (5.7)

which means that the adhesive will wet the surface of the adherend when the surface free energy of the substrate is greater.

Usually the determination of surface free energies of solids can be made by mea-suring the contact angles of appropriate probe liquids on the solid surface. Different contact analysis techniques are applied in the measurements of various forms of sub-strates. One is the sessile drop method (Figure 5.10) which is also referred to as static contact angle technique. The angle can also be calculated by the following equation with obtained height (h) and radius (r) of the spherical drop.

θ

−=

+

12 2

2( )

rhContact angle sin

r h (5.8)

Figure 5.9 Schematic representation of LBL coating of wood fibers with polyethyleneimine (PEI), poly(sodium 4-styrenesulfonate (PSS) and indium tin oxide (ITO) [57].


Another method is the Wilhelmy Plate technique that is suitable for making contact angle measurements on thin plates and single fibers. According to Figure 5.11, the contact angle can be calculated using the Wilhelmy equation (Equation 5.9) [60].

F = gLP cos q + mg – rL Ahg (5.9)where:

F=advancing or receding force on the sample in liquidgL = surface tension of the liquid P = perimeter of the wetted cross sectionm = mass of the specimeng = acceleration due to gravityρL = liquid densityA = cross-sectional area of the specimenh = depth of immersion.For particles (also fibers), by recording the process of liquid going through a column

attributed to capillary forces where particles of interest are packed inside, the contact angle

Figure 5.10 Schematic illustration of the sessile drop method.

Figure 5.11 Schematic illustration of the Wilhelmy method.


can be calculated from the Washburn equation (Equation 5.10) [61] that governs the wick-ing process:

λ θ

η=

2 cos2LtR

h

(5.10)

where: h = height to which liquid has risen as a function of time t R = effective interstitial pore radius between the packed particlesgL = surface tension of the liquidη = viscosity of the liquid. Recently a three-dimensional dynamic contact angle analysis method was developed

that can determine the key parameters such as volume and contact angle during sessile droplet measurement [62]. A strong point of this method is that even when the orientation of the droplet’s axis changes with time, the dynamic parameters of droplet on anisotropic surfaces can be determined. However, one limitation of this technique is that because the model is based on spherical and ellipsoidal droplets, the calculated volumes and contact angles for non-elliptical oblong droplets are not reliable.

The methods for determining surface free energy of solids based on contact angles are various, for example the Zisman approach [63], the Neumann (the equation of state) [64], the Chibowski approach [65], the harmonic mean approach [66], Owens and Wendt approach (the geometric mean) [67] and the acid-base approach [10], which are reported in a recent review by Etzler [65]. Most of the methods have already been applied to wood surface adhesion. Another way to compute surface free energy of fibers or particles is by inverse gas chromatography (IGC) which provides an alternative to the measurement of contact angles [68]. Regarding the wetting of wood materials, there are several influen-tial factors: wood species, sapwood or heartwood, grain orientation, wood elements and chemical composition, surface roughness, absorption and capillary flow, sample aging, and machine test speed, different treatments, etc. [69–72].

5.4.1 Wood Anatomy Impact on Wetting

In general, hardwoods have lower contact angles than softwoods whose surfaces contain more hydrophobic substances, such as resins and unsaturated fatty acids, in addition to phe-nolic compounds in both species [73]. As reported, huge variability in surface free energy of wood exists along the stem [74]. The polar component of surface free energy is found to be high at the stem base and gradually decreases with height, but the dispersion component acts in an opposite manner. Because the polar component of surface free energy is mark-edly higher than the dispersion component, in general, the surface free energy decreases along the tree height. No significant difference in surface free energy is observed across the tree cross section. Wood is more easily wetted along the grain direction than across the grain direction [75, 76]. This results from the fact that the liquid drop placed on the surface


will spread easily along the wood cell lumen in the grain direction than across the lumen because of capillary effects during the adhesive wetting process [70].

Heartwood and sapwood have different influences on the wetting by adhesives. Heartwood normally contains extractives and hardened parenchyma cells. Additionally, closed pits in heartwood will greatly reduce the number of microscale pathways for adhe-sive penetration. Therefore, the heartwood is expected to be less wettable than sapwood in terms of instantaneous liquid spreading. However, the difference in equilibrium contact angles between heartwood and sapwood depends on the wood species and resin type [70]. On the microscale, earlywood consists of parenchyma and is distorted more by cutting tools, resulting in protruding cell fragments. Therefore, earlywood is often rougher than latewood [77]. So normally, wettability and surface free energy of earlywood are greater than latewood.

5.4.2 Extractives

The influence of extractives on the wettablility of wood surface is significant because of both physical blocking of adhesive penetration and chemically affecting the curing process [78, 79]. Decreased wettability can also be attributed to the change in wood chemical com-position [80] and acid-base property of the wood surface when it is covered by extractives [81]. Because the surface composition and surface free energy of the solid are altered, the measured contact angle is not representative of the state of the surface of interest. A detailed description of the effect of extractives on wood surface wettability can be found in the sec-tion on weak boundary layers.

5.4.3 Adhesive Wettability

The differences among contact angles are related to the nature of the adhesives. PF resins are more hydrophobic than UF resins because of the phenyl rings present in their structures. As a result, the contact angles of PF resins should be larger than that of UF resins. On the other hand, viscosity of UF and PF is also an important factor affecting wetting [82]. The contact angle increases with an increase in resin viscosity [83]. Wetting of soy protein adhesives modified by urea on wood surfaces was found to increase because the molecular attraction between the adhesive and the wood surface was greater than that between adhesive mol-ecules after modification [83]. Incorporation of dendritic compounds which bear different end groups such as -OH, -NH2, and NH3

+, Cl- into urea-formaldehyde adhesive has been shown to be effective adhesion promoters for wood bonding. This promotion is partially attributed to the improved wetting caused by the large number of polar functional groups present at the interface [84].

5.4.4 Wood Modification

Within a short period of time, freshly-cut wood surfaces undergo a transformation that has been referred to as surface inactivation. The major reason for the change of wood surface


free energy is the migration of low molecular weight extractives to the wood surface and the oxidation of extractives thereafter. This will reduce the surface wettability and impair the bonding of wood elements. Several chemical, physical, and mechanical methods can be applied to activate the surface from hydrophobic to hydrophilic, which is further dis-cussed in the weak boundary layer section. At the same time, a hydrophobic wood is usu-ally favored since hydrophilicity is an intrinsic property of wood, which is detrimental to physical properties, mechanical properties, and utility and service life of wood-based prod-ucts. To minimize the hydrophilic effect, acetylation and grafting of hydrophobic groups on wood are general methods. Besides the interaction with moisture, flammability and biodegradability are intrinsic properties of wood that hamper its application. These can be overcome or ameliorated by fire retardant and preservative treatments. For bonding wood to fiber-reinforced plastics (FRPs), hydroxymethyl resorcinol (HMR) is used as adhesion promoter to produce durable adhesive bonds in an exterior environment for a number of diverse resin adhesive systems [85]. All these modifications will affect the wettability and surface free energy of wood and a brief review is given below. A summary of these treat-ments on the wettability of wood is shown in Table 5.6.

5.4.4.1 Acetylation

Acetylation is basically an esterification process where the available hydroxyl groups of wood are reacted with acetic anhydride. A review on acetylation of wood was reported by Rowell [86]. Chemicals that have been studied include acids (carboxylic acids, phthal-dehydic acids), aldehydes (formaldehyde, difunctional aldehydes, acetaldehyde, chloral), chlorides (acid chlorides, alkyl chlorides), esters (β-propiolactone), ketene, isocyanates, acrylonitrile, dimethyl sulfate and epoxides. It was reported that all of these chemicals can create a hydrophobic layer on the wood surface by reacting with hydroxyls, reducing the wetting of polar adhesives on wood [87].

5.4.4.2 Grafting

Grafting is referred to as the covalent bonding of various substances like polymers or mon-omers, as well as smaller organic or inorganic compounds onto functional groups on a surface. For wood surfaces, new functional groups are introduced by reacting with existing functional groups (mainly hydroxyl) in wood polymers. A grafting process can be achieved by plasma, wet chemical and even enzymatic methods. Wettability of treated wood surfaces is determined by the functional groups grafted onto the wood. Hydrophobic groups will reduce the wetting of wood [88–90]. Prolonging the reaction duration of grafting will add more polymers on the wood substrate, creating a less wettable surface.

5.4.4.3 Fire Retardants, Preservatives and Adhesion Promotion

Ayrilmis et al. (2009) studied the wettability of fire retardant treated laminated veneer lum-ber (LVL) with a borax-boric acid compound, monoammonium phosphate and diammo-nium phosphate [91]. They found that waterborne fire retardants such as boron compounds and phosphates can make more wettable wood surfaces attributed to the hygroscopic


characteristics of the reagents, which was confirmed by other researchers [92]. On the other hand, other research indicated that borax and boric acid used as preservative resulted in poor wettability of alder and beech veneer surfaces [93]. The authors pointed out that wet-tability is dependent on the type of impregnation chemical, and also the wood species and resin used for bonding wood. The effects of two wood preservatives (waterborne chromated copper arsenate (CCA) and organometallic copper naphthenate (CuN)) on the surface free energy of wood and E- glass/ phenolic composite material were also investigated [94]. The CuN treatment increased the contact angle of water on southern yellow pine because of the oily non-polar nature of this kind of preservative. CCA treatment reduced the contact angle of water and increased the total surface free energy of wood by metallic salts with high surface energy.

Hydroxymethyl resorcinol (HMR) can produce durable adhesive bonds for application in exterior environments. The dynamic adhesive wettability of HMR treated wood with PF and PMDI was found to decrease because the dispersion component of surface free energy decreased in this treatment [85]. The principles and details of wood surface modifications can be found in a recent review by Petric [95].

5.4.5 Test Methods

Compared to homogeneous materials like plastics and metals, the wetting process of wood is more complex and is attributed to its intrinsic surface properties like heterogeneity, roughness, and liquid adsorption, which makes the thermodynamic equilibrium condi-tions assumed by Young’s equation not applicable to wood surface [96]. Therefore, some modifications on contact angle measurements and calculations are applied by researchers to take into account these factors. The sessile drop contact angle technique also referred to as static contact angle measurement has appeared in most previous studies on determination of the thermodynamics of the liquid/solid interaction of wood where the instantaneous or equilibrium contact angles were usually used [97]. However, it has been reported that time is a significant factor affecting the measured surface parameters of the wood surface [75, 98, 99]. Thus, simply comparing initial or equilibrium contact angles is not as meaningful as investigating all other phenomena during the wetting process like spreading and penetration [70]. Models and parameters (decay ratio, spreading ratio and changing rates) can be used to illustrate the dynamic wetting process [99].

During the measurement of contact angles on wood surfaces by the Wilhelmy plate method, a hysteresis phenomenon is observed [71]. That is, the advancing contact angle on the wood surface during the immersion process is usually higher than the receding contact angle when the liquid is withdrawn from the wood surface. This phenomenon is attributed to two reasons: (1) wood surface is heterogeneous which represents the low-energy sites during advancing contact angle measurement, while the surface transforms to high-energy sites during receding angle measurement because it is already covered with the test liquid; (2) molecules or hydroxyl groups in the solid surface rearrange after contact with a polar liquid. For instance, hydroxyl groups on the wood surface try to avoid contact with inert air by hiding under the surface, but they reorient to form hydrogen bonds when the surface

144 Progress in Adhesion and AdhesivesTa

ble

5.6

Cha

nge

in w

etta

bilit

y aft

er d

iffer

ent t

reat

men

ts o

n w

ood

surf

aces

.

Trea

tmen

tSp

ecie

sC

onta

ct a

ngle

Surf

ace

free

en

ergy

W

ettin

gR

efer

ence

s

Ace

tyla

tion

Scot

s pin

e (P

inus

sylv

estr

is)↑

↓ (a

cid

-bas

e)↓

[87]

Gra

fting

Pine

(Pin

ussil

vestr

is)↑

↓ (a

cid–

base

)↓

[90]

Fire

reta

rdan

tBe

ech

(Fag

us o

rient

alis

lipsk

y)↓

-↑

[91]

Pres

erva

tive

Ald

er (A

lnus

glut

inos

a su

bsp.

ba

rbat

a)Be

ech

(Fag

us o

rient

alis)

Sout

hern

yel

low

pin

e

↑ (B

oric

aci

d)↑

Cop

per N

apht

hena

te

(CuN

)↓

Chr

omat

ed C

oppe

r A

rsen

ate

(CC

A)

-↓(

Boric

aci

d)↓(

CuN

)↑(

CC

A)

[93]

[94]

(Adh

esio

n pr

omot

erH

ydro

xym

ethy

l re

sorc

inol

) (H

MR)

Sout

hern

pin

e (P

inus

sp

p.) a

nd D

ougl

as-fi

r (P

seud

otsu

ga m

enzi

esii)

↓↑

(aci

d -b

ase)

↓[2

06]

↑Inc

reas

e, ↓D

ecre

ase,

acid

-bas

e: a

cid-

base

com

pone

nt o

f sur

face

free

ene

rgy


is in contact with water [71, 100]. End-sealing wood veneers or polymeric foams with poly (vinyl acetate) is recommended as a standard practice for Wilhelmy plate analysis because it can reduce capillary uptake of the probe liquid, reducing premature wetting of the wood surface. Another way to minimize the effect of capillarity is to increase the test speed to reduce the time for absorption.

It is not accurate to use column wicking measurements to determine the contact angle on swelling polymer substrates by the Washburn equation because the effective interstitial pore radius (R) in the wicking experiments changes after the particles swell. Two phenom-ena observed during the wicking process indicate that the equation needs further modifi-cation: (1) heat was released during the wicking process when the polar liquids contacted the swelling polymer particles, and (2) compared to using hexane (non-polar liquid), using methanol (polar liquid) resulted in a smaller average capillary radius (r) [101]. At the same time, Shi and Gardner (2000) reported one attempt to account for swelling and energy loss during the wicking tests [101].

5.5 Diffusion Theory of Adhesion

The diffusion theory is based on the concept that two materials are soluble in one another, i.e. compatible, and if they are brought into close contact, they dissolve in one another and form an interphase which is a solution of both materials in one another and therefore does not form a discontinuity of physical properties between the two materials (Figure 5.12) [6]. In the case of wood adhesive bonding, this theory is more applicable for an adhesive that diffuses/penetrates into the wood cell wall. Penetration is the ability of an adhesive to move into the voids on the surface of a substrate or into the substrate itself. The cellular nature of wood can cause significant penetration of an adhesive into the substrate. Adhesive penetra-tion follows the path of least resistance into the porous structure, either by gross penetra-tion or by cell wall penetration [102]. According to Frihart (2004) there are four scenarios of penetration: 1) occupation of free volume, 2) mechanical interlocking, 3) interpenetrat-ing polymer networks, which form cross-links between the adhesive molecules within the free volume of the cell wall, and 4) chemical cross-links with cell wall polymers [103].

Figure 5.12 Schematic of diffusion theory of adhesion: (a) two compatible materials are brought into close contact (b) and an interphase (c) is formed where both materials mix and/or entangle with one another.


Adhesive bond performance between wood elements is presumed to be significantly influenced by the degree of penetration of the adhesive into the porous network of intercon-nected cells and is controlled by 1) adhesive variables (molecular weight (MW), distribu-tion), 2) substrate variables (species, orthogonal plane, moisture content) and 3) processing variables (curing method i.e. time, pressure, and temperature). Swelling is defined as the uptake of liquid molecules by wood polymers in the wood cell wall and is driven by dif-fusion. The degree of swelling decreases with increasing size of the molecules in the liq-uid [104]. Mantanis et al. (1994) showed that even relatively large molecules, like pyridine (MW=79) and benzyl alcohol (MW= 108), are able to swell the wood cell wall significantly because of their high capacity to form hydrogen bonds [104]. Furuno et al. (2004) studied the penetration of PF resin at different MWs (290, 470, 820) into the wood cell wall and found that low MW resin penetrated more into the cell wall whereas the medium and high MW resins penetrated only slightly [105]. A lower MW resin is also more likely to contrib-ute to dimensional stability and decay resistance [105]. Similar results were reported by Laborie and coworkers [106, 107] suggesting a nanoscale miscibility of low MW PF resin with wood polymers using solid-state NMR spectroscopy.

Research investigating bondline performance has focused on methods using micro-scopic examination and associated techniques with the goal of establishing a relationship between penetration and bond performance. Modzel et al. (2011) provide a good review on microscopic methods used and resin systems studied to examine adhesive penetration in wood [108]. Figure 5.13 gives an overview of the most commonly used methods and examples of attainable images to study resin penetration into wood.

Figure 5.13 Photomicrographs of PF adhesive bondlines in hybrid poplar (a-d) and Douglas-fir (e-h) with rubidium replacing sodium in the PF formulation for better detection. Images show the same field of view for each species and were created using fluorescence microscopy (a+e), scanning electron microscopy (SEM) (b+f), SEM with back-scatter detector (BSE) (c+g), and SEM with wavelength-dispersive spectroscopy (WDS) (d+h). Images courtesy of Fred Kamke.


More recent work using micro- or synchrotron radiation X-ray computed tomography has shown its advantage over more conventional microscopic techniques in providing an accurate representation of the 3D microstructure of the wood and penetrating adhesive system [109–111]. Furthermore, it is possible to calculate the stress and strain in the adhe-sive bondline using a micromechanical model proposed by Kamke et al. [112]. Besides the adhesive type, especially the characteristics of the wood elements to be bonded play an important role in penetration and bond performance. Gruver and Brown (2006) investi-gated the penetration and bond performance of pMDI according to: 1) species, 2) anatom-ical bonding plane, and 3) moisture content by fluorescence microcopy and compression shear block test [113]. While a species and corresponding anatomical effect was observed, overall bond performance was mostly influenced by moisture content. The influence of species and corresponding anatomy is most apparent in hardwood species, where the pen-etration occurs mostly in the vessel network compared to softwoods where the penetration is more evenly distributed and interconnected [110]. Mendoza et al. (2012) developed a model to predict adhesive penetration into the interconnected vessel network of hard-woods taking into account adhesive hardening, capillary penetration, and processing tech-niques [114]. A different method to characterize resin penetration is described by Wang and Yan [115] who used mercury intrusion porosimetry to measure the pore volume with and without resin. Their work showed that processing parameters are an important factor, since pressure applied by hot- pressing promoted resin penetration into smaller pores.

While gross penetration into wood cells is relatively easy to study, the methods employed are often limited by their resolution and/ or contrast capabilities to investigate cell wall penetration on the nano- or even angstrom scale. To date, the most successful methods to investigate cell wall penetration use some form of tagging of the adhesive to make the tag detectable by the method employed. The penetration of UF/UMF resin into MDF fiber walls was shown by means of confocal scanning microcopy in combination with staining [116, 117]. Xing et al. (2005) observed that the penetration was directional through the outside of the fiber wall towards the lumen and not horizontal along cell wall layers, pre-sumably through pores in the cell wall [116]. Gindl and coworkers (2002, 2003) used UV absorbance microscopy specifically for melamine in MUF and MF resin to study cell wall penetration. Their results confirm that resin diffusion into the wood microstructure occurs for these adhesive types [118, 119]. The UV absorbance microscopy method was further used to study the effect of resin diffusion on the stability of adhesively bonded joints of PUR and PRF resins on cell wall mechanical properties by nanoindentation [120]. PRF resin penetrated the cell wall microstructure while PUR resin did not. The elastic modulus deter-mined by nanoindentation of resin infiltrated undamaged cells did not change significantly but hardness increased. The presence of PRF but not of PUR and epoxy resin in wood cell walls was also determined by scanning thermal microscopy [121].

Another more recently developed technique is the use of solid or solution state NMR spectroscopy to investigate polymer miscibility and therefore nano scale cell wall penetra-tion of isotopic labeled PF and pMDI resins [107, 122–124]. NMR spectroscopy has also found application in investigating the adhesiveless bonding of wood by high speed rotation welding [125]. The mechanism for high speed rotation, as well as linear vibration welding,


relies on mechanical friction and corresponding temperature-induced softening and pen-etration of amorphous polymer material (mostly hemicelluloses and lignin) into the inter-cellular structure of the wood. The result is a high densification of the bonded interphase, yielding high quality wood-wood joints [125–127].

5.6 Covalent Bonding

A covalent bond is a bond where two atoms share an electron pair and is believed to improve the bond durability between wood and an adhesive. While covalent bonds occur in certain fields of adhesion their existence was for a long time not believed to occur between wood and adhesives [11, 128]. It is well established that the energy of activation of the condensa-tion of wood adhesives (e.g. PF, UF, MF and MUF) is influenced by the presence of wood polymers compared to neat resin [129–132]. According to Pizzi et al. (1994) two effects are present when an adhesive cures on a wood surface, 1) catalytic self-activation of the resin self-condensation induced by carbohydrates and 2) formation of resin-substrate covalent bonds induced by lignin [130]. However, the contribution of the second one is very small and often negligible under the conditions pertaining to thermosetting adhesive applications [130, 131] and do not exist for MUF systems [133].

A limiting factor for the investigation of resin to wood substrate covalent bonding is the difference between conditions in the laboratory and industrial settings. Often used differ-ential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) techniques are subject to time and temperature limitations since heating rates are much slower (10–20°C/min) compared to a maximum heating and curing of only a few minutes in industrial com-posite board production processes. Even more extreme sampling requirements are neces-sary when infrared (IR) and ultraviolet (UV) spectrophotometries are used [130].

Most of the more recent research in wood-adhesive covalent bonding focuses on phe-nyl isocyanate-based adhesives, since they are most likely to form urethane (or carbamate) bonds with wood polymers and are shown to penetrate the wood cell wall and intimately associate with wood molecules [122–124, 132, 134]. Solid-state NMR spectroscopy can be used to study urethane formation using isotopic labeled isocyanate pMDI. However, this method cannot identify covalent bonds with certainty since urethane and polyurea signals display a significant overlap in the acquired spectra and are of low signal intensity [123, 124, 134]. A solution-state NMR spectroscopy technique was further developed to allow for a more accurate determination of isocyanate reactivity with wood. While this method was able to detect urethane formation between phenyl isocyanate and cell wall polymers no covalent bonds could be determined in experiments under typical conditions used for industrial oriented strand board (OSB) bonding using pMDI [135–137]. Bao et al. (2003) proposed a model for pMDI bonding and attribute the strength of this particular wood-adhesive bond to the deep penetration of adhesive molecules into the wood cell wall and middle lamella and the crosslinking between individual sections between and within cells [138]. All together it is very unlikely that covalent bonds form in any significant amount under conditions characteristic of thermosetting wood adhesive applications, even though


the formation of covalent resin-substrate bonds has been demonstrated to exist. If the advantages of covalent wood-adhesive bonds are desired for certain applications, future work in this field needs to focus on adjusting processing parameters to allow for the forma-tion of covalent bonds and further development of detection methods.

5.7 Acid-base Theory

Based on the correlation of acid-base interactions by Drago et al. (1971) [139], Fowkes and Mostafa proposed a new method to interpret the interactions during polymer adsorption where the polar interaction is referred to as an acid-base interaction [140]. In this interac-tion, an acid (electron-acceptor) is bonded to a base (electron-donor) by sharing the elec-tron pair offered by the latter, which forms a coordinate bond.

Thomas Young formulated (without using equations) the relation between the surface tension of a liquid and a solid, the interfacial tension between the solid and the liquid, and the contact angle θ for a drop of liquid deposited on a flat horizontal surface [141]. This formulation is usually expressed as Young’s equation:

cosLG SG SLγ θ γ γ= − (11)

where: gLG and gSG are the surface free energies of the liquid and the solid, respectively, exposed to a gas (G), and gSL is the solid-liquid interfacial free energy.

Perhaps the most convenient way of interpreting the wettability of a low-energy surface, such as that of various polymers or ligno-cellulosic materials, is by the formulation of the work of adhesion, Wa, defined as the work required to separate unit area of the solid-liquid interface [142], i.e.

Wa = gS + gL – gSL (5.12)where: gS and gL are the surface free energies of the solid (S) and liquid (L) surfaces in vacuum, and gSL is the solid-liquid interfacial free energy.

The surface free energy per unit area (or surface tension) gi of the substance i is defined as half the work of cohesion Wc, i.e.:

12i cWγ = (5.13)

Assuming that gL ≈ gLG and gS ≈ gSG, combination of equations (5.11) and (5.12) leads to the Young-Dupré equation:

(1 cos )a LW γ θ= + (5.14)

Hence, if the contact angle, θ, of a well-defined probe liquid against a solid is measured, the work of adhesion can be determined.


The following descriptions briefly summarize the Lewis acid-base concept in wetting-related phenomena. According to Fowkes [143] and van Oss et al. [144], the total work of adhesion in interfacial interaction between solids and liquids can be expressed as the sum of the Lifshitz-van der Waals (LW) and the Lewis acid-base (AB) interactions, viz.

LW AB

a a aW W W= + (5.15)

The separation of the work of adhesion into LW and AB components is also applicable to the surface free energies according to:

LW AB

i i iγ γ γ= + (5.16)

A breakthrough in the understanding of wetting phenomena was the Good-Girifalco-Fowkes ‘geometric mean’ combination rule for the LW interactions between two com-pounds i and j, which can be expressed as [145, 146]:

γ γ= 2LW LW LWa i jW (5.17)

Hence, if the q is determined for both a non-polar and a polar liquid, with known g LW param-eters, on the same surface, then Wa

LW and WaAB can be determined using equations (5.14–5.17).

The acid-base theory plays a critical role in surface chemistry and adhesion and it has been exploited broadly on different materials [147, 148]. Several models of calculating the surface energy of solids were proposed where acid-base theory was applied, including Fowkes’s method, Good’s method, van Oss’s method and Chang’s method [65]. In terms of surface energy of wood determined by contact angle analysis, results from the Zisman approach deviate from other methods; the equation of state method is practical to an adhe-sion interaction only having a dispersion component because the polar part of the equa-tions is not related to contact angle. For chemically heterogeneous materials like wood, the acid-base approach is a more valuable method because it can provide the most detailed information about the surface chemistry including the values of Lifshitz-van der Waals (dispersion) and Lewis acid-base (polar) components of surface free energy. Because the type of contact angle test liquid has an influence on the results of the acid-base approach, a minimum of three liquids is recommended for this method [10, 149].

Analysis of wood extractives reveals that they are the dominant factor influencing the acid-base properties [81]. Wood surfaces with extractives exhibit less acidic (electron-accepting) character and greater basic (electron-donating) character and the Lifshitz-van der Waals (dispersion) surface free energy component increases after the elimination of extractives from spruce wood particles [68].

5.8 Weak Boundary Layer

Bikerman first introduced the concept of a weak boundary layer (WBL) in adhesion sci-ence in 1961 [150]. In his model, three general classes of WBLs were specified, i.e., air


bubbles, impurities at the interface, and reactions between components and the medium. It was pointed out that the interface was the place where the failure of a bonded assembly will occur when a weak boundary layer was present. The WBL is important because people initially thought that an interface between an adhesive and a substrate would not fail, but actually it can fail if a WBL is present. Thus, to achieve satisfactory adhesive bonding per-formance, the weak boundary layer should be eliminated.

In regards to wood adhesion, Stehr and Johansson gave a detailed definition of the WBL (Figure 5.14) [151]. They classified wood weak boundary layers into two groups. The first group is referred to as a chemical weak boundary layer (CWBL) which is attributed to low-molecular-weight compounds on the wood surface. This contamination usually comes from extractives of wood, containing both hydrophobic and hydrophilic components. The second group is referred to as a mechanical weak boundary layer (MWBL) formed by dam-age to the wood surface. The MWBL derives from surfaces going through machining pro-cesses (planing, sanding, peeling, etc.), weathering (light delignification) and other forms of loose particles (dirt), as well as trapped air bubbles.

Christiansen (1990) focused on the influence of excessive drying (surface inactivation) on wood bonding. Physical responses to excessive drying include: 1) migration of extrac-tives to the surface, lowering the wettability or covering the surface, 2) rearrangement of wood surface molecules, reducing wettability or sites for bonding, and 3) permanent clo-sure of large micropores in cell walls [78]. Chemical responses to excessive drying include: 1) deteriorating wood surface strength, 2) degrading wood surface by oxidation and pyrol-ysis reactions, 3) chemical interference with resin cure or bonding, and 4) reducing the number of surface hydroxyl groups by etherification [79]. Besides, drying methods can have different impacts on wettability of wood [69]. For example, research found that polar components of oven and rotating-drum dried strands were greater than that of air and microwave dried strands [152].

5.8.1 Extractives

The mechanisms proposed for surface inactivation appear valid to some degree. The wood aging effect is generally attributed to the exudation of wood extractives to the exterior sur-faces after machining processes, which decreases the wettability and surface free energy of wood surfaces [98, 153]. Overall, the chemical compositions of extractives have much lower oxygen to carbon (O/C) ratios compared with cellulose, causing a hydrophobic effect, which is detrimental to the wetting of polar adhesives [80, 154]. The hydrophobic compo-nents from bark extractives have more difficulty in being covered by the polar adhesive during gluing. Thus, bark extractives are adverse to the interaction of the particle/phenol-formaldehyde adhesive system [155].

Extractives often affect the curing process of adhesives because they contain both acidic and basic constituents. For PF resin, some previous studies on the effects of wood extrac-tives on the curing reaction of resole (alkaline curing) resins showed that the acidity of the wood extractives inhibited the curing of the resole by changing them from weak basic to neutral, which resulted in the formation of a large quantity of dimethylene ether link-ages [156, 157]. A definitive answer pertaining to the influence of wood extractives and


their chemical compositions on the hardening behavior of aminoplastic condensation resin adhesives (urea- and melamine-formaldehyde) is not clear: acceleration as well as retarda-tion of the hardening process have been reported [158, 159]. Acceleration of the hardening process of aminoplastic resins usually results from a decrease in the adhesive pH after appli-cation onto a wood surface [160]. Resin curing retardation arises from the basic component in wood extractives. However, dissimilar to the former two cases, extractives of different wood species from cold water extraction had almost no effect on the curing process of UF resin and melamine-modified UF resin [158]. For wood-cement composites, hydration of cement was inhibited by low molecular weight carbohydrates and hemicelluloses [161].

Extractives also have a negative influence on the hydrogen bonding prevalent in wood pellet manufacturing [162, 163]. Detailed explanation was given that extractives block many binding sites on the particles, therefore, little hydrogen bonding or van der Waals forces occur between particles, forming a chemical weak boundary layer [164, 165]. A positive aspect of extractives during wood pellet production is that they have a beneficial effect on lowering the energy consumed during compression by acting as plasticizers and lubricants [162].

5.8.2 Heat Treatment

Heat-treated woods have outdoor applications attributed to their enhanced properties such as reduced hygroscopicity, improved dimensional stability, and improved resistance to degradation by microorganisms. Much research has focused on the change in wettability of heat-treated woods because excessive heat can cause problems during varnish or paint application. The wettability of wood decreases after high temperature treatment [166–172]. Contact angles on heat-treated wood surfaces increase because of a more hydrophobic character of the surface attributed to partial degradation of hemicelluloses and the plasti-cization of lignin, leading to a rearrangement of the wood polymer components [173–179] and exudation of extractives and VOC-like substances to the surface [80]. In contrast, an interesting result was that certain exterior waterborne coatings were found to exhibit much better wetting on oil-heat-modified Scots pines than on unmodified samples that should be more hydrophilic and readily wetted. It was believed that surface tensions of the tested waterborne coatings were low, attributed to the increased hydrophobicity by adding silicone

Figure 5.14 Revised scheme for weak boundary layer in wood adhesion [151].


and wax into the coating and the dispersion component of the surface free energy increased after heat treatment [180].

5.8.3 Wood Impregnation and Densification

After the first patented procedures appeared, wood densification methods have been avail-able for at least a century [172]. Wood can be densified because it is constituted of cells whose lumens can be filled with other materials like polymers, wax, sulfur and molten met-als. The surface energy of densified wood decreases slightly compared to hyro-thermally treated wood [181], which may be attributed to the hydro-thermal treatment induced dur-ing the densification process [182].

5.8.4 Machining Processes

It is difficult to generalize whether a rough wood surface has a positive or negative impact on adhesive bonding performance. On one hand, roughness is a disadvantage because it contains failure initiation sites and hinders adhesives and coatings to penetrate into the wood substrate [183, 184] and anchor to intact wood material [185]. On the other hand, roughness appears, to some extent, to improve the adhesive joint performance [186] because crushed fibers can mechanically interlock with adhesives [187] and offers a larger contact area on the surface of wood [82]. Moreover, a rougher surface can provide enhanced cap-illary forces and expose more porous structures in the wood [25]. Wood-adhesive bond performance deteriorates only when the damaged fibers become excessive and unattached.

Different machining methods applied in the process of making wood elements have various effects on the surface wettability. Overall, regardless of the tools used, an increase in surface roughness enhances wettability and adhesion. In contrast to sanding and face milling, helical planing produced higher surface roughness on birch wood, with better wet-ting properties [188]. Compared to peripheral knife planing, the sanding process on sugar maple wood caused cell wall fibrillation and only a few open vessels, while planed surfaces were smoother and consisted of more open cells. Thus, the surfaces of sanded wood were rougher than planed and more hydroxyl groups were displayed. As a result, the sanded wood surface has a higher surface energy and wettability [77]. Face-milled black spruce wood had more subsurface damage, fibrillation, and open lumens that favored coating pen-etration than oblique cutting and helical planing generated surfaces [189].

Wettability probe liquids most readily wet sanded surfaces, which have higher surface roughness and capillary forces compared with sawed, planed, and razor blade cut south-ern pine surfaces [183]. In addition, planing homogenizes surfaces, while sanding activates surfaces by mechanical cell modification [73]. However, since sanding redistributes non-wettable extractives on the wood surface, surface tension of sanded wood may decrease more rapidly with time than surface tension of microtome prepared or planed wood [98]. Planing parameters (rake angle and feed speed) have a significant effect on surface energy and the total surface energy is strongly influenced by the dispersion component of surface energy [190]. As pointed out by Cheng and Sun (2006), the final adhesive bonding property


of wood products depends on wettability which sometimes acts as a secondary effect, and also on processing factors such as curing time and pressure [25]. Therefore, comprehensive consideration is needed when dealing with bonding performance of an adhesive bonded wood product [25].

5.8.5 Surface Degradation

In the high temperature environment of wood element processing such as drying, it was speculated that a small amount of active hydroxyl groups were consumed in carboxyla-tion reactions instead of reacting with phenolic adhesives [191]. Wood dried in a nitrogen environment is less inactivated than wood dried in air based on the values of bond strength and wood failure, which is a proof for oxidation occurring during drying. As temperature increased, the difference between the characteristic oxidation times of white spruce became smaller, indicated by the result from infrared spectroscopy measured in the atmosphere of nitrogen and air. The observation suggests that pyrolysis reactions are the predominant source of surface degradation.

Ugovšek et al. (2012) applied liquefied wood as an adhesive on the surface of beech (Fagus sylvatica L.) and used light microscopy, scanning electron microscopy, FT-IR micro-spectroscopy, and elemental carbon, nitrogen and sulfur (CNS) analysis techniques to inves-tigate adhesive bonding [192]. The results showed that a layer of partially delignified cells, defined as a weak boundary layer, existed between the original wood cells on the adherend and carbonized cells. Because lignin isassumed to be a fortifying polymer of wood cells, the strength of the cell wall was dramatically weakened and bond shear strengths were relatively low.

Weathering is an important process affecting wood surface wetting. Wettability often increases after weathering since cracks propagate and a more hydrophilic surface is formed by transforming the crystalline area into an amorphous area [193, 194].

5.8.6 Surface Activation

Elimination of extractives usually results in wetting improvement, but it is also species dependent. For hot water extraction of red maple, this enhancement is attributed to the loss of lipophilic substances, higher pore volume created during the extraction process and increased acid-base characteristics from the hemicellulose cleavage which exposes high sur-face energy functional groups [195]. Douglas-fir was more wettable after extraction because the dispersion force attributed to low or nonpolar extractives was predominant before extrac-tion, while after extraction, the polar force became relatively prevalent [196]. However, post-drying extraction by a variety of different organic solvents such as acetone, petroleum ether and benzene-ethanol did not increase the wettability of high-temperature-dried Douglas-fir veneer [197], which may be attributed to the wood surface reacting with fatty acids and forming ether bonds with the cellulose during the high temperature drying [198]. Boiling water extraction of oak chips before bonding increases internal bond (IB) strength and bend-ing strength, which can also be achieved by sodium carbonate treatment [199].


Exposure of a wood surface to UV light is a simple and effective way to improve the wet-tability of the surface [200]. UV light can open the pits (spruce), change surface morphol-ogy and alter surface chemical composition to a certain extent. Because of the oxidative activation effect, wettability and carbonyl group concentration of bamboo surface increased with UV irradiation time [201].

5.9 Discussion and Future Research Prospects

Upon reviewing the literature for this paper, it became apparent that there were several wood adhesion theories that deserve greater attention because of the lack of a body of knowledge on the subject. The electrostatic theory of adhesion has received little scientific attention over the last half century relative to wood adhesion probably because it works well for coating applications and thus was readily accepted in the commercial marketplace. It is difficult for researchers to solicit and obtain financial backing for fundamental research on a topic that is well accepted in the commercial marketplace. However, there may be applica-tions of electrostatic adhesion that could benefit the production of bonded wood materials especially in wood nanomaterial applications.

The contact mechanics between surfaces or particles and a surface remains an interest-ing topic for researchers. Many studies have been done on contact mechanics starting in late 1800’s with Hertz and followed by DMT and JKR. Nowadays, with the newest state-of-the-art measurement equipment, new methods are used to measure the forces between sur-faces. AFM is one of the most popular techniques to measure the adhesion forces between two surfaces, which uses force- deflection curves and calculates adhesion forces according to these nanomechanical interactions. A study investigating the adhesive bonding proper-ties of wood- plastic composite (WPC) and continuous glass fiber reinforced (FPR) surfaces using AFM for measuring the surface roughness and adhesion forces is a good example [202]. On the other hand, nanoindentation is another novel technique which works with the same principle (force- deflection curves) as AFM but nanoindentation techniques give more accurate results because the setup is capable of applying the force to the surface with 90° while creating force- deflection curves. When the knowledge gained from previous studies is integrated with the newest measurement techniques, there will be less assump-tions and more accurate studies on adhesion forces, and these measurements will be capa-ble of addressing smaller length scales and adhesion forces.

The covalent or chemical bonding theory applied to wood adhesion is a challenging area because of the complex nature of both wood and the adhesives used to bond wood. This bonding mechanism has been addressed by a number of researchers, but many questions regarding this topic remain unanswered. Perhaps with newer imaging techniques and more sophisticated solid state chemistry analytical methods, the question of covalent bonding in wood adhesive applications can be addressed with greater certainty. The ability to create suc-cessful nonreversible covalent bonds between wood and an adhesive would ultimately lead to extremely durable wood adhesion for adverse environmental applications of wood products.

Ongoing research in studying wood adhesion theories using more easily accessible and tried and true techniques such as wettability and contact angle analysis, optical- and


electron microscopic techniques, thermal analysis, spectroscopic techniques as well as novel techniques like AFM and nanoindentation will continue. Following are some thoughts on future prospects for research on wood adhesion.

The effects of wood anatomical characteristics on wood wettability have already received considerable attention. Based on the articles cited in this review, certain aspects of wood wettability may need further investigation. For example, the different influences of tracheids in softwoods and vessel elements in hardwoods on wood wettability deserve more detailed study because they have different pore sizes that impact adhesive-wood interactions and subsequent penetration. Related articles discussed the wetting variations between along the grain direction and across the grain direction. However, even along the grain direction, there are still two distinguished faces (radial and tangential). Different amounts of cell wall microstructural inclusions (pits) exist on these faces, which will affect the penetration of adhesive into the wood cellular structure. Few articles have dealt with surface wettability of woods after modification by chemical reagents, where wettability becomes very important if the samples need further adhesive bonding or painting. In these cases, the wood modi-fication processes may have significant effects on wood surface properties and wettability should be studied. For its heterogeneous nature, the contact angle measurements on the wood surface are quite complicated compared to homogeneous materials. Modifications have been adopted in the methods of contact angle measurements on wood which take the hygroscopic and heterogeneous characteristics of wood into account and certain improve-ments are observed. The modified and even original contact angle calculation equations contain simplifications and assumptions that are not necessarily accurate, which can lead to inaccurately calculated contact angles. Moreover, the prevalent surface free energy com-putations are based on the measured contact angles, thus an imprecise contact angle will result in surface free energy deviations that may lessen the significance of these methods. Therefore, more refined techniques are required to make contact angle measurements. From this perspective, IGC may be a better choice for surface free energy analyses because it is based on the affinities between column packing material and different probe gases, avoiding the inherent problems with contact angle measurements. Although IGC is rela-tively simple and accurate, its ability to obtain surface free energy is not perfect because the diluted gases are usually adsorbed on high energy sites during the testing of heterogeneous materials like wood whose mechanism should be explored in future research.

There are many constituents in wood extractives, which can influence the adhesion between wood and an adhesive. However, not all extractives have negative effects on adhe-sion. For example, cold water extraction had little or no effect on the curing process of UF and MUF resin. Though some research pointed out that lipophilic component of wood extractives interfered with adhesion, relationships are still unknown between the specific types of extrac-tives and their influences on wood adhesion parameters, like formerly mentioned acid-base property, chemical compositions, number of hydrogen bonding sites, curing process of adhe-sives, etc. It will be beneficial to analyze the extractives’ compositions and correlate them to adhesion.

One suggestion is that effort should be exerted on figuring out the influence of wood extractives and their chemical compositions on the hardening behavior of aminoplastic


condensation resin adhesives which is not entirely clear now. This is important to decide whether an extraction procedure is necessary for an aminoplastic resin system.

Regarding the effect of heat treatment on wood adhesion, the problem is that most pre-vious papers dealing with this issue usually did not exclude the influence of extractives which also have the effect of reduced wetting of the wood surface as a result of heat treat-ment. Extractives may mask the mechanism of decreased wettability attributed only to heat treatment. The measured decreased surface free energy of densified wood has the same problem because this process is often associated with heat and moisture, and altered sur-face properties cannot be attributed solely to densification. Besides, the surface property of wood densified with materials like polymers and metals is not readily seen in past research, however, this is important because it is related to secondary processing like painting and overlaying laminates.

For the surface roughness caused by different machining methods, it is hard to gener-alize which method can produce the roughest surface because the roughness is not only dependent on the machining methods and processing parameters, but also on other factors such as wood species and anatomical characteristics. Therefore, if one wants to study the effects of different machining methods on wood surface properties, it is critical to make sure that other factors are the same. In addition, although roughness is beneficial to a product’s mechanical properties as a result of mechanical interlocking with an adhesive, excessive wood cell wall damage can result in deteriorated bonding. Optimized surface roughness should be determined to maintain the roughness at a reasonable level. Combined with the roughness caused by different machining methods for certain species of woods, it should be possible to determine optimal machining process characteristics for each particular wood species.

The novel techniques of atomic force microscopy and nanoindentation can be used to determine adhesion forces and pull-off forces between wood and adhesives. The surface properties, pores, crevices, roughness of wood on the nanolevel should be investigated using these techniques. New adhesion models should be developed according to more accurately determined geometric properties of the wood surface. Future studies should be more focused on the effect of ambient conditions on adhesion forces at the nano level and determining the optimum conditions for adhesion. In addition to this, the adhesion between a single cellulose nanofiber and different adhesives should be studied in depth and the adhesion system based on a single cellulose nanofiber structure should be explained and compared to wood as a bonding substrate. As a result of these unique studies, the methods and conditions that will enhance the adhesion between cellulose nanofibers and adhesives can be determined and described in detail.

5.10 Summary

The study of wood adhesion theories has and will continue to be an important topic for researchers, and practitioners of wood adhesive bonding. The topics of wood wettability and the weak boundary layer have tended to dominate the study of wood adhesion over the past several decades. The diffusion theory of wood is beginning to receive greater attention


with the availability of improved analytical capabilities. Mechanical interlocking has long been accepted as a wood adhesion mechanism but is receiving new focus as researchers examine the nanometer length scale of wood-adhesive interactions. Both the electrostatic and covalent bonding theories deserve greater research attention from the wood research community. It is envisioned that much of the new knowledge being generated regarding wood adhesion theories will be incremental in nature unless researchers adopt non-con-ventional approaches and experimental methodologies to address this subject area. Are breakthroughs in understanding wood adhesion coming in the near term?

References

1. A. Pizzi, Guest editorial special issue on “Wood Adhesives”. J. Adhesion Sci. Technol. 24, 1353–1355 (2010).

2. C.R. Frihart, Wood adhesion and adhesives, in Handbook for Wood Chemistry and Wood Composites, R.M. Rowell (Ed.), pp. 215–278, CRC Press, Boca Raton, FL. (2012).

3. Forest Products Laboratory, Wood Handbook: Wood as an Engineering Material, p. 508. General Technical Report FPL-GTR-190. U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI. (2010).

4. F.H. Chung, Unified theory and guidelines on adhesion. J. Appl. Polym. Sci. 42, 1319–1331 (1991).

5. A. Baldan, Adhesion phenomena in bonded joints. Int. J. Adhesion Adhesives 38, 95–116 (2012). 6. A.V. Pocius, Adhesion and Adhesives Technology—An Introduction, 3rd edition, p. 370, Carl

Hanser Verlag, München. (2012). 7. J. Schultz and M. Nardin, Theories and mechanisms of adhesion, in Handbook of Adhesive

Technology, A. Pizzi and K.L. Mittal (Eds.), pp. 19–33, Marcel Dekker, New York. (1994). 8. J. Custódio, J. Broughton, and H. Cruz, A review of factors influencing the durability of struc-

tural bonded timber joints. Int. J. Adhesion Adhesives 29, 173–185 (2009). 9. C.R. Frihart, Adhesive groups and how they relate to the durability of bonded wood. J. Adhesion

Sci. Technol. 23, 601–617 (2009). 10. D.J. Gardner, Application of the Lifshitz-van der Waals acid-base approach to determine wood

surface tension components. Wood Fiber Sci. 28, 422–428 (1996). 11. D.J. Gardner, Adhesion mechanisms of durable wood adhesive bonds, in Characterization

of the Cellulosic Cell Wall, D.D. Stokke and L.H. Groom (Eds.), pp. 254–265, John Wiley & Sons. (2005).

12. A.J. Panshin and C.de Zeeuw, Textbook of Wood Technology, 4th edition, p. 722, McGraw-Hill, New York. (1980).

13. D.D. Stokke and D.J. Gardner, Fundamental aspects of wood as a component of thermoplastic composites. J. Vinyl Additive Technol. 9, 96–104 (2003).

14. D.J. Gardner, M.P. Wolcott, L. Wilson, Y. Huang, and M. Carpenter, Our understanding of wood chemistry, in Proc. 1995 Wood Adhesive Symposium, pp. 29–36, Forest Products Society, Madison, WI. (1996).

15. V. Gray, The wettability of wood. Forest Prod. J. 12, 452–461 (1962). 16. A.A. Marra, Technology of Wood Bonding: Principles and Practice, p. 454, Van Nostrand

Reinhold, New York. (1992). 17. D.J. Gardner, G.S. Oporto, R. Mills, and M.A.S.A. Samir, Adhesion and surface issues in cel-

lulose and nanocellulose. J. Adhesion Sci. Technol. 22, 545–567 (2008).


18. C. Miao and W.Y. Hamad, Cellulose reinforced polymer composites and nanocomposites: A critical review. Cellulose 20, 2221–2262 (2013).

19. J.W. McBain and D.J. Hopkins, On adhesives and adhesive action. J. Phys. Chem. 29, 188–204 (1925).

20. H. Weiss, Adhesion of advanced overlay coatings: Mechanisms and quantitative assessment. Surface Coatings Technol. 71, 201–207 (1995).

21. S. Joo and D.F. Baldwin, Adhesion mechanisms of nanoparticle silver to substrate materials: Identification. Nanotechnology 21, 055204 (2010).

22. A.N. Gent and C.W. Lin, Model studies of the effect of surface roughness and mechanical inter-locking on adhesion. J. Adhesion 32, 113–125 (1990).

23. B.S. Gupta, I. Reiniati, and M.G. Laborie, Surface properties and adhesion of wood fiber rein-forced thermoplastic composites. Colloids Surfaces 302, 388–395 (2007).

24. W.C. Wake, Adhesion and the Formulation of Adhesives, p. 332, Applied Science Publishers, London. (1982).

25. E. Cheng and X. Sun, Effects of wood-surface roughness, adhesive viscosity and processing pressure on adhesion strength of protein adhesive. J. Adhesion Sci. Technol. 20, 997–1017 (2006).

26. W. Kim, I. Yun, L. Jung and H. Jung, Evaluation of mechanical interlock effect on adhesion strength of polymer-metal interfaces using micro-patterned topography. Int. J. Adhesion Adhesives 30, 408–417 (2010).

27. J. Follrich, A. Teischinger, W. Gindl and U. Müller, Effect of grain angle on shear strength of glued end grain to flat grain joints of defect-free softwood timber. Wood Sci. Technol. 41, 501–509 (2007).

28. M.G. Salemane and A.S. Luyt, Thermal and mechanical properties of polypropylene-wood powder composites. J. Appl. Polym. Sci. 100, 4173–4180 (2006).

29. M.J. Smith, H. Dai and K. Ramani, Wood-thermoplastic adhesive interface - Method of charac-terization and results. Int. J. Adhesion Adhesives 22, 197–204 (2002).

30. M. Kazayawoko, J.J. Balatinecz and L.M. Matuana, Surface modification and adhesion mecha-nisms in woodfiber-polypropylene composites. J. Mater. Sci. 34, 6189–6199 (1999).

31. C.B. Vick and T.A. Kuster, Mechanical interlocking of adhesive bonds to CCA-treated Southern pine - A scanning electron microscopic study. Wood Fiber Sci. 24, 36–46 (1992).

32. A.C. Backman and K.A.H. Lindberg, Interaction between wood and polyvinyl acetate glue studied with dynamic mechanical analysis and scanning electron microscopy. J. Appl. Polym. Sci. 91, 3009–3015 (2004).

33. J. Drelich and K.L. Mittal (Eds.), Atomic Force Microscopy in Adhesion Studies, CRC Press, Boca Raton, FL. (2005).

34. K.L. Johnson, K. Kendall, and A.D. Roberts, Surface energy and the contact of elastic solids. Proc. R. Soc. Lond. A. 324, 301–313 (1971).

35. D.S. Grierson, E.E. Flater, and R.W. Carpick, Accounting for the JKR-DMT transition in adhe-sion and friction measurements with atomic force microscopy. J. Adhesion Sci. Technol. 19, 291–311 (2005).

36. C. Morrow, M. Lovell, and X. Ning, A JKR-DMT transition solution for adhesive rough surface contact. J. Phys. D: Appl. Phys. 36, 534–540 (2003).

37. C.S. Hodges, L. Looi, J.A.S. Cleaver, and M. Ghadiri, Use of the JKR model for calculating adhe-sion between rough surfaces. Langmuir 20, 9571–9576 (2004).


38. W.C. Oliver and G.M. Pharr, Improved technique for determining hardness and elastic modu-lus using load and displacement sensing indentation experiments. J. Mater. Res. 7, 1564–1583 (1992).

39. J.E. Jakes, C.R. Frihart, J.F. Beecher, R.J. Moon, and D.J. Stone, Experimental method to account for structural compliance in nanoindentation measurements. J. Mater, Res. 23, 1113–1127 (2008).

40. J. Konnerth and W. Gindl, Mechanical characterization of wood-adhesive interphase cell walls by nanoindentation. Holzforschung 60, 429–433 (2006).

41. B.V. Derjaguin, I.N. Aleinikova, and Y.P. Toporov, On the role of electrostatic forces in the adhe-sion of polymer particles to solid surfaces. Powder Technol. 2, 154–158 (1969).

42. A.G. Bailey, The science and technology of electrostatic powder spraying, transport and coat-ing. J. Electrostatics 45, 85–120 (1998).

43. M.N. Horenstein, Electrostatics and nanoparticles: What’s the same, what’s different? J. Electrostatics 67, 384–393 (2009).

44. I.I. Inculet, Electrostatics in industry. J. Electrostatics 4, 175–192 (1978). 45. D. Rimai, K. Brown, M. Zaretsky, K. Lofftus, M. Aslam, W. Fowlkes, and D. Weiss, The role of

adhesion in electrophotographic digital printing. J. Adhesion Sci. Technol. 24, 583–617 (2010). 46. L. Schein, W.S. Czarnecki, B. Christensen, T. Mu, and G. Galliford, Experimental verification of

the proximity theory of toner adhesion. J. Imaging Sci. Technol. 48, 417–425 (2004). 47. S. Di Risio and N. Yan, Bioactive paper through inkjet printing. J. Adhesion Sci. Technol. 24,

661–684 (2010). 48. M.J. Dvorchak, Using high performance two-component waterborne polyurethane wood coat-

ings. J. Coating Technol. 69, 47–52 (1997). 49. E.M. Ransburg, Electrostatic coating apparatus, US Patent 2684656 (1954). 50. W.A. Starkey, Electrostatic coating, US Patent 2723921 (1955). 51. T. Misev and R. Van der Linde, Powder coatings technology: New developments at the turn of

the century. Prog. Org. Coat. 34, 160–168 (1997). 52. M.A. Hubbe, Bonding between cellulosic fibers in the absence and presence of dry-strength

agents—A review. BioResources 1, 281–318 (2007). 53. K. Ariga, J.P. Hill, M.V. Lee, A. Vinu, R. Charvet, and S. Acharya, Challenges and breakthroughs

in recent research on self-assembly. Sci. Technol. Adv. Mater. 9, 014109 (2008). 54. L. Wägberg, S. Forsberg, A. Johansson, and P. Juntti, Engineering of fibre surface properties by

application of the polyelectrolyte multilayer concept. Part I: Modification of paper strength. J. Pulp Paper Sci. 28, 222–228 (2002).

55. M. Agarwal, Y. Lvov, and K. Varahramyan, Conductive wood microfibres for smart paper through layer-by-layer nanocoating. Nanotechnology 17, 5319–5325 (2006).

56. Z. Lu, S. Eadula, Z. Zheng, K. Xu, G. Grozdits, and Y. Lvov, Layer-by-layer nanoparticle coatings on lignocellulose wood microfibers. Colloids Surfaces 292, 56–62 (2007).

57. C. Peng, Y. Thio, and R. Gerhardt, Conductive paper fabricated by layer-by-layer assembly of polyelectrolytes and ITO nanoparticles. Nanotechnology 19, 505603 (2008).

58. S. Renneckar and Y. Zhou, Nanoscale coatings on wood: Polyelectrolyte adsorption and layer-by-layer assembled film formation. ACS Appl. Mater. Interfaces 1, 559–566 (2009).

59. K.L. Mittal, The role of the interface in adhesion phenomena. Polym. Eng. Sci. 17, 467–473 (1977).

60. L. Wilhelmy, Über die Abhängigkeit der Capillaritätskonstanten des Alkohols von Substanz und Gestalt des benetzten festen Körpers. Annalen der Physik 195, 177–217 (1863).


61. E.W. Washburn, The dynamics of capillary flow. Phys. Rev. 17, 273 (1921). 62. D. Baptista, L. Muszyński, D.J. Gardner, and E. Atzema, An experimental method for

three-dimensional dynamic contact angle analysis. J. Adhesion Sci. Technol. 26, 2199–2215 (2012).

63. W.A. Zisman, Influence of constitution on adhesion. Ind. Eng. Chem. Res. 55, 18–38 (1963). 64. A. Neumann, R. Good, C. Hope and M. Sejpal, An equation-of-state approach to determine

surface tensions of low-energy solids from contact angles. J. Colloid Interface Sci. 49, 291–304 (1974).

65. F.M. Etzler, Determination of the surface free energy of solids: A critical review. Rev. Adhesion Adhesives 1, 3–45 (2013).

66. S.H. Wu, Calculation of interfacial tension in polymer systems. J. Polym. Sci. C: Polym. Symp. 34, 19–30 (1971).

67. D.K. Owens and R. Wendt, Estimation of the surface free energy of polymers. J. Appl. Polym. Sci. 13, 1741–1747 (1969).

68. M.E. Wålinder and D.J. Gardner, Surface energy of extracted and non-extracted Norway spruce wood particles studied by inverse gas chromatography (IGC). Wood Fiber Sci. 32, 478–488 (2000).

69. T.F. Shupe, C.Y. Hse, and W.H. Wang, An investigation of selected factors that influence hard-wood wettability. Holzforschung 55, 541–548 (2001).

70. S.Q. Shi and D.J. Gardner, Dynamic adhesive wettability of wood. Wood Fiber Sci. 33, 58–68 (2001).

71. D.J. Gardner, N.C. Generalla, D.W. Gunnells, and M.P. Wolcott, Dynamic wettability of wood. Langmuir 7, 2498–2502 (1991).

72. C. Piao, J.E. Winandy, and T.F. Shupe, From hydrophilicity to hydrophobicity: A critical review: Part I. Wettability and surface behavior. Wood Fiber Sci. 42, 490–510 (2010).

73. I. Santoni and B. Pizzo, Effect of surface conditions related to machining and air exposure on wettability of different mediterranean wood species. Int. J. Adhesion Adhesives 31, 743–753 (2011).

74. D. Rossi, S. Rossi, H. Morin, and A. Bettero, Within-tree variations in the surface free energy of wood assessed by contact angle analysis. Wood Sci. Technol. 46, 287–298 (2012).

75. S. Lee, T.F. Shupe, L.H. Groom, and C.Y. Hse, Wetting behaviors of phenol- and urea-formalde-hyde resins as compatibilizers. Wood Fiber Sci. 39, 482–492 (2007).

76. Ü. Büyüksarı, T. Akbulut, C. Guler, and N. As, Wettability and surface roughness of natural- and plantation grown narrow leaved ash (Fraxinus angustifoilia vahl.) wood. BioResources 6, 4721–4730 (2011).

77. L.F. de Moura and R.E. Hernández, Evaluation of varnish coating performance for two surfac-ing methods on sugar maple wood. Wood Fiber Sci. 37, 355–366 (2005).

78. A.W. Christiansen, How overdrying wood reduces its bonding to phenol-formaldehyde adhe-sives: A critical review of the literature. Part I. Physical responses. Wood Fiber Sci. 22, 441–459 (1990).

79. A.W. Christiansen, How overdrying wood reduces its bonding to phenol-formaldehyde adhe-sives: A critical review of the literature. Part II. Chemical reactions. Wood Fiber Sci. 23, 69–84 (1991).

80. M. Šernek, F.A. Kamke, and W.G. Glasser, Comparative analysis of inactivated wood surfaces. Holzforschung 58, 22–31 (2004).

81. M.E. Wålinder and D.J. Gardner, Acid–base characterization of wood and selected thermoplas-tics. J. Adhesion Sci. Technol. 16, 1625–1649 (2002).


82. I. Aydin, G. Colakoglu, and S. Hiziroglu, Surface characteristics of spruce veneers and shear strength of plywood as a function of log- temperature in peeling process. Int. J. Solids Struct. 43, 6140–6147 (2006).

83. H.-N. Xu, Q.-Y. Shen, X.-K. Ouyang, and L.-Y. Yang, Wetting of soy protein adhesives modified by urea on wood surfaces. Eur. J. Wood Wood Prod. 70, 11–16 (2012).

84. H.A. Essawy and H.A. Mohamed, Poly (amidoamine) dendritic structures, bearing differ-ent end groups as adhesion promoters for urea-formaldehyde wood adhesive system. J. Appl. Polym. Sci. 119, 760–767 (2011).

85. D.J. Gardner, C.E. Frazier, and A.W. Christiansen, Characteristics of the wood adhesion bond-ing mechanism using hydroxymethyl resorcinol, in Proceedings of the Wood Adhesives 2005 Conference, pp. 93–97, Forest Products Society, San Diego, CA. (2005)

86. R.M. Rowell, Acetylation. Forest Prod. J. 56, 4–12 (2006). 87. L.E. Bryne and M.E. Wålinder, Ageing of modified wood. Part 1: Wetting properties of acety-

lated, furfurylated, and thermally modified wood. Holzforschung 64, 295–304 (2010). 88. Y. Fu, G. Li, H. Yu, and Y. Liu, Hydrophobic modification of wood via surface-initiated ARGET

ATRP of MMA. Appl. Surface Sci. 258, 2529–2533 (2012). 89. M.H. Drafz, S. Dahle, W. Maus-Friedrichs, J.C. Namyslo, and D.E. Kaufmann, Chemical

improvement of surfaces. Part 2: Permanent hydrophobization of wood by covalently bonded fluoroorganyl substituents. Holzforschung 66, 727–733 (2012).

90. M.Ł. Mamiński, K. Mierzejewska, P. Borysiuk, P. Parzuchowski, and P. Boruszewski, Surface properties of octadecanol—grafted pine veneers. Int. J. Adhesion Adhesives 29, 781–784 (2009).

91. N. Ayrilmis, T. Dundar, Z. Candan, and T. Akbulut, Wettability of fire retardant treated laminated veneer lumber (LVL) manufactured from veneers dried at different temperatures. BioResources 4, 1536–1544 (2009).

92. R.H. Alamjuri, A. Zaidon, F. Abood, and U.K. Anwar, Adhesion and bonding characteristics of preservative-treated bamboo (Gigantochloa scortechinii) laminates. J. Appl. Sci. 10, 1435–1441 (2010).

93. I. Aydin and G. Colakoglu, Variation in surface roughness, wettability and some plywood properties after preservative treatment with boron compounds. Build. Environ. 42, 3837–3840 (2007).

94. C. Tascioglu, B. Goodell, R. Lopez-Anido, and D. Gardner, Surface energy characterization of preservative-treated wood and E-glass/phenolic composites. Forest Prod. J. 54, 262–268 (2004).

95. M. Petrič, Surface modification of wood. Rev. Adhesion Adhesives 1, 216–247 (2013). 96. M. de Meijer, S. Haemers, W. Cobben, and H. Militz, Surface energy determinations of wood:

Comparison of methods and wood species. Langmuir 16, 9352–9359 (2000). 97. T. Nguyen and W.E. Johns, Polar and dispersion force contributions to the total surface free

energy of wood. Wood Sci. Technol. 12, 63–74 (1978). 98. M. Gindl, A. Reiterer, G. Sinn, and S.E. Stanzl-Tschegg, Effects of surface ageing on wettability,

surface chemistry, and adhesion of wood. Holz Roh Werkst. 62, 273–280 (2004). 99. J.Z. Lu and Q. Wu, Surface characterization of chemically modified wood: Dynamic wettability.

Wood Fiber Sci. 38, 497–511 (2006).100. M.E. Wålinder and G. Ström, Measurement of wood wettability by the Wilhelmy method. Part

2. Determination of apparent contact angles. Holzforschung 55, 33–41 (2001).101. S.Q. Shi and D.J. Gardner, A new model to determine contact angles on swelling polymer parti-

cles by the column wicking method. J. Adhesion Sci. Technol. 14, 301–314 (2000).102. F.A. Kamke and J.N. Lee, Adhesive penetration into wood—A review. Wood Fiber Sci. 39, 205–

220 (2007).


103. C. R. Frihart, Adhesive Interactions with Wood, in Fundamentals of Composite Processing. Proceedings of a Workshop, Forest Products Laboratory, Madison, WI, Gen. Tech. Rep. FPL-GTR-149, p. 118 (2004).

104. G.I. Mantanis, R.A. Young, and R.M. Rowell, Swelling of wood. Part II. Swelling in organic liquids. Holzforschung 48, 480–490 (1994).

105. T. Furuno, Y. Imamura, and H. Kajita, The modification of wood by treatment with low molecu-lar weight phenol-formaldehyde resin: A properties enhancement with neutralized phenolic-resin and resin penetration into wood cell walls. Wood Sci. Technol. 37, 349–361 (2004).

106. M.-P.G. Laborie, L. Salmén, and C.E. Frazier, A morphological study of the wood/phenol-for-maldehyde adhesive interphase. J. Adhesion Sci. Technol. 20, 729–741 (2006).

107. M.-P.G. Laborie and C.E. Frazier, 13C CP/MAS NMR study of a wood/phenol–formaldehyde resin bondline. J. Mater. Sci. 41, 6001–6005 (2006).

108. G. Modzel, F. Kamke, and F. De Carlo, Comparative analysis of a wood: Adhesive bondline. Wood Sci. Technol. 45, 147–158 (2011).

109. J.L. Paris, F.A. Kamke, R. Mbachu, and S.K. Gibson, Phenol formaldehyde adhesives formulated for advanced x-ray imaging in wood-composite bondlines. J. Mater. Sci. 49, 1–12 (2013).

110. P. Hass, F. Wittel, M. Mendoza, H. Herrmann, and P. Niemz, Adhesive penetration in beech wood: Experiments. Wood Sci. Technol. 46, 243–256 (2012).

111. G. Standfest, S. Kranzer, A. Petutschnigg, and M. Dunky, Determination of the microstructure of an adhesive-bonded medium density fiberboard (MDF) using 3D sub-micrometer computer tomography. J. Adhesion Sci. Technol. 24, 1501–1514 (2010).

112. F.A. Kamke, J.A. Nairn, L. Muszynski, J.L. Paris, M. Schwarzkopf, and X. Xiao, Methodology for micromechanical analysis of wood adhesive bonds using X-ray computed tomography and numerical modeling. Wood Fiber Sci. 46, 1–14 (2014).

113. T.M. Gruver and N.R. Brown, Penetration and performance of isocyanate wood binders on selected wood species. BioResources 1, 233–247 (2006).

114. M. Mendoza, P. Hass, F. Wittel, P. Niemz, and H. Herrmann, Adhesive penetration of hard-wood: A generic penetration model. Wood Sci. Technol. 46, 529–549 (2012).

115. W. Wang and N. Yan, Characterizing liquid resin penetration in wood using a mercury intru-sion porosimeter. Wood Fiber Sci. 37, 505–513 (2005).

116. C. Xing, B. Riedl, A. Cloutier, and S.M. Shaler, Characterization of urea–formaldehyde resin penetration into medium density fiberboard fibers. Wood Sci. Technol. 39, 374–384 (2005).

117. P.-L. Cyr, B. Riedl, and X.-M. Wang, Investigation of urea-melamine-formaldehyde (UMF) resin penetration in medium-density fiberboard (MDF) by high resolution confocal laser scan-ning microscopy. Holz Roh Werkst. 66, 129–134 (2008).

118. W. Gindl, E. Dessipri, and R. Wimmer, Using UV-microscopy to study diffusion of melamine-urea-formaldehyde resin in cell walls of spruce wood. Holzforschung 56, 103–107 (2002).

119. W. Gindl, F. Zargar-Yaghubi, and R. Wimmer, Impregnation of softwood cell walls with mela-mine-formaldehyde resin. Bioresource Technol. 87, 325–330 (2003).

120. W. Gindl, T. Schöberl, and G. Jeronimidis, The interphase in phenol–formaldehyde and pol-ymeric methylene di-phenyl-di-isocyanate glue lines in wood. Int. J. Adhesion Adhesives 24, 279–286 (2004).

121. J. Konnerth, D. Harper, S.-H. Lee, T.G. Rials, and W. Gindl, Adhesive penetration of wood cell walls investigated by scanning thermal microscopy (SThM). Holzforschung 62, 91–98 (2008).

122. S. Das, M.J. Malmberg, and C.E. Frazier, Cure chemistry of wood/polymeric isocyanate (PMDI) bonds: Effect of wood species. Int. J. Adhesion Adhesives 27, 250–257 (2007).


123. S.L. Wendler and C.E. Frazier, The effects of cure temperature and time on the isocyanate-wood adhesive bondline by 15N CP/MAS NMR. Int. J. Adhesion Adhesives 16, 179–186 (1996).

124. X. Zhou and C.E. Frazier, Double labeled isocyanate resins for the solid-state NMR detection of urethane linkages to wood. Int. J. Adhesion Adhesives 21, 259–264 (2001).

125. A. Pizzi, A. Despres, H.-R. Mansouri, J.-M. Leban, and S. Rigolet, Wood joints by through-dowel rotation welding: Microstructure, 13C-NMR and water resistance. J. Adhesion Sci. Technol. 20, 427–436 (2006).

126. B. Gfeller, M. Zanetti, M. Properzi, A. Pizzi, F. Pichelin, M. Lehmann, and L. Delmotte, Wood bonding by vibrational welding. J. Adhesion Sci. Technol. 17, 1573–1589 (2003).

127. A. Zoulalian and A. Pizzi, Wood-dowel rotation welding—A heat-transfer model. J. Adhesion Sci. Technol. 21, 97–108 (2007).

128. W.E. Johns, The chemical bonding of wood, in Wood Adhesives, Chemistry and Technology, Vol. 2, A. Pizzi (Ed.), pp. 75–96, Marcel Dekker, New York. (1989).

129. H. Mizumachi and H. Morita, Activation energy of the curing reaction of phenolic resin in the presence of woods. Wood Sci. Technol. 7, 256–260 (1975).

130. A. Pizzi, B. Mtsweni, and W. Parsons, Wood-induced catalytic activation of PF adhesives autopolymerization vs. PF/wood covalent bonding. J. Appl. Polym. Sci. 52, 1847–1856 (1994).

131. G. He and B. Riedl, Curing kinetics of phenol formaldehyde resin and wood-resin interactions in the presence of wood substrates. Wood Sci. Technol. 38, 69–81 (2004).

132. J.J. Marcinko, S. Devathala, P.L. Rinaldi, and S. Bao, Investigating the molecular and bulk dynamics of PMDI/wood and UF/wood composites. Forest Prod. J. 48, 81–84 (1998).

133. A. Pizzi and L.A. Panamgama, Diffusion hindrance vs. wood-induced catalytic activation of MUF adhesive polycondensation. J. Appl. Polym. Sci. 58, 109–115 (1995).

134. S. Bao, W.A. Daunch, Y. Sun, P.L. Rinaldi, J.J. Marcinko, and C. Phanopoulos, Solid state NMR studies of polymeric diphenylmethane diisocyanate (PMDI) derived species in wood. J. Adhesion 71, 377–394 (1999).

135. D.J. Yelle, J.E. Jakes, J. Ralph, and C.R. Frihart, Characterizing polymeric methylene diphe-nyl diisocyanate reactions with wood: 1. High-resolution solution-state NMR spectroscopy, in Proceedings of the Wood Adhesives 2009 Conference, Lake Tahoe, NV, pp. 338–342, Forest Products Socienty, Madison, WI. (2010).

136. D.J. Yelle, A Solution-State NMR Approach to Elucidating pMDI-Wood Bonding Mechanisms in Loblolly Pine, PhD thesis, University of Wisconsin, Madison, WI (2009).

137. D.J. Yelle, J. Ralph, and C.R. Frihart, Delineating pMDI model reactions with loblolly pine via solution-state NMR spectroscopy. Part 2. Non-catalyzed reactions with the wood cell wall. Holzforschung 65, 145 (2011).

138. S. Bao, W.A. Daunch, Y. Sun, P.L. Rinaldi, J.J. Marcinko, and P. Chris, Solid state two-dimen-sional nmr studies of polymeric diphenylmethane diisocyanate (pmdi) reaction in wood. Forest Prod. J. 53, 63 (2003).

139. R.S. Drago, G.C. Vogel, and T.E. Needham, Four-parameter equation for predicting enthalpies of adduct formation. J. Am. Chem. Soc. 93, 6014–6026 (1971).

140. F.M. Fowkes and M.A. Mostafa, Acid-base interactions in polymer adsorption. Ind. Eng. Chem. Prod. R+D 17, 3–7 (1978).

141. T. Young, An essay on the cohesion of fluids. Philos. Trans. Roy. Soc. 95, 65–87 (1805).142. A. Dupré, Théorie Mécanique de la Chaleur, p. 585. Gustav Zeuner, Gauthier-Villars, Paris.

(1869).


143. F.M. Fowkes, Acid-base interactions in polymer adhesion, in Physicochemical Aspects of Polymer Surfaces, Vol. 2, K.L. Mittal (Ed.), pp. 583–603, Plenum Press, New York. (1983).

144. C.J. van Oss, M.K. Chaudhury, and R.J. Good, Monopolar surfaces. Adv. Colloid Interface Sci. 28, 35–64 (1987).

145. R.J. Good and L.A. Girifalco, A theory for estimation of surface and interfacial energies, III. Estimation of surface energies of solids from contact angle data. J. Phys. Chem. 64, 561–565 (1960).

146. F.M. Fowkes, Additivity of intermolecular forces at interfaces. I. Determination of the contribu-tion to surface and interfacial tension of dispersion forces in various liquids. J. Phys. Chem. 67, 2538–2541 (1963).

147. K.L. Mittal (Ed.), Acid-Base Interactions: Relevance to Adhesion Science and Technology, Vol. 2, CRC Press, Boca Raton, FL. (2000).

148. K.L. Mittal and H.R. Anderson, Jr. (Eds.), Acid-Base Interactions: Relevance to Adhesion Science and Technology, CRC Press, Boca Raton, FL. (1991).

149. M. Gindl, G. Sinn, W. Gindl, A. Reiterer, and S. Tschegg, A comparison of different methods to calculate the surface free energy of wood using contact angle measurements. Colloids Surfaces 181, 279–287 (2001).

150. J.J. Bikerman, The Science of Adhesive Joints, p. 258, Academic Press, New York. (1961).151. M. Stehr and I. Johansson, Weak boundary layers on wood surfaces. J. Adhesion Sci. Technol.

14, 1211–1224 (2000).152. S. Wang, Y. Zhang, and C. Xing, Effect of drying method on the surface wettability of wood

strands. Holz Roh Werkst. 65, 437–442 (2007).153. F. Englund, L.E. Bryne, M. Ernstsson, J. Lausmaa, and M. Wålinder, Spectroscopic studies of

surface chemical composition and wettability of modified wood. Wood Mater. Sci. Eng. 4, 80–85 (2009).

154. P. Widsten, V.S. Gutowski, S. Li, T. Cerra, S. Molenaar, and M. Spicer, Factors influencing timber gluability with one-part polyurethanes – Studied with nine Australian timber species. Holzforschung 60, 423–428 (2006).

155. M. Claude, N. Yemele, A. Koubaa, P. Diouf, P. Blanchet, A. Cloutier, and T. Stevanovic, Effects of hot-water treatment of black spruce and trembling aspen bark raw material on the physical and mechani-cal properties of bark particleboard. Wood Fiber Sci. 40, 339–351 (2008).

156. E. Alamsyah, M. Yamada, and K. Taki, Bondability of tropical fast-growing tree species III: Curing behavior of resorcinol formaldehyde resin adhesive at room temperature and effects of extractives of Acacia mangium wood on bonding. J. Wood. Sci. 54, 208–213 (2008).

157. I. Abe and K. Ono, Effect of the acidity of some tropical wood extractives on the curing of the resol. J. JWRS 26, 686–692 (1980).

158. B. Stefke and M. Dunky, Catalytic influence of wood on the hardening behavior of formalde-hyde-based resin adhesives used for wood-based panels. J. Adhesion Sci. Technol. 20, 761–785 (2006).

159. C.-Y. Hse and M.-L. Kuo, Influence of extractives on wood gluing and finishing—A review. Forest Prod. J. 38, 52–56 (1988).

160. G. Nemli, E.D. Gezer, S. Yıldız, A. Temiz, and A. Aydın, Evaluation of the mechanical, physical properties and decay resistance of particleboard made from particles impregnated with Pinus brutia bark extractives. Bioresource Technol. 97, 2059–2064 (2006).

161. M. Fan, M.K. Ndikontar, X. Zhou, and J.N. Ngamveng, Cement-bonded composites made from tropical woods: Compatibility of wood and cement. Const. Build. Mater. 36, 135–140 (2012).


162. N.P.K. Nielsen, D.J. Gardner, and C. Felby, Effect of extractives and storage on the pelletizing process of sawdust. Fuel 89, 94–98 (2010).

163. N.P.K. Nielsen, D.J. Gardner, T. Poulsen, and C. Felby, Importance of temperature, moisture content, and species for the conversion process of wood residues into fuel pellets. Wood Fiber Sci. 41, 414–425 (2009).

164. R. Samuelsson, S.H. Larsson, M. Thyrel, and T.A. Lestander, Moisture content and storage time influence the binding mechanisms in biofuel wood pellets. Appl. Energy 99, 109–115 (2012).

165. W. Stelte, J.K. Holm, A.R. Sanadi, S. Barsberg, J. Ahrenfeldt, and U.B. Henriksen, A study of bonding and failure mechanisms in fuel pellets from different biomass resources. Biomass Bioenergy 35, 910–918 (2011).

166. J. Follrich, U. Müller, and W. Gindl, Effects of thermal modification on the adhesion between spruce wood (Picea abies karst.) and a thermoplastic polymer. Holz Roh Werkst. 64, 373–376 (2006).

167. B. Esteves and H. Pereira, Wood modification by heat treatment: A review. BioResources 4, 370–404 (2008).

168. N. Ayrilmis and J.E. Winandy, Effects of post heat-treatment on surface characteristics and adhesive bonding performance of medium density fiberboard. Mater. Manuf. Process. 24, 594–599 (2009).

169. O. Unsal, Z. Candan, U. Buyuksari, S. Korkut, and M. Babiak, Effects of thermal modification on surface characteristics of OSB panels. Wood Res. 55, 51–58 (2010).

170. Z. Candan, U. Büyüksarı, S. Korkut, O. Unsal, and N. Çakıcıer, Wettability and surface rough-ness of thermally modified plywood panels. Ind. Crop. Prod. 36, 434–436 (2012).

171. Ü. Büyüksarı, Surface characteristics and hardness of MDF panels laminated with thermally compressed veneer. Composites Part B 44, 675–678 (2013).

172. U. Aleš, F.A. Kamke, M. Sernek, M. Pavlič, and A. Kutnar, The wettability and bonding perfor-mance of densified VTC beech (Fagus sylvatica l.) and Norway spruce (Picea abies (l.) karst.) bonded with phenol–formaldehyde adhesive and liquefied wood. Eur. J. Wood Wood Prod. 71, 371–379 (2013).

173. M. Pétrissans, P. Gérardin, I.E. Bakali, and M. Serraj, Wettability of heat-treated wood. Holzforschung 57, 301 (2003).

174. M. Hakkou, M. Pétrissans, A. Zoulalian, and P. Gérardin, Investigation of wood wettability changes during heat treatment on the basis of chemical analysis. Polym. Degrad. Stabil. 89, 1–5 (2005).

175. P. Gérardin, M. Petrič, M. Petrissans, J. Lambert, and J.J. Ehrhrardt, Evolution of wood surface free energy after heat treatment. Polym. Degrad. Stabil. 92, 653–657 (2007).

176. D. Kocaefe, S. Poncsak, G. Doré, and R. Younsi, Effect of heat treatment on the wettability of white ash and soft maple by water. Holz Roh Werkst. 66, 355–361 (2008).

177. A. Kutnar, F.A. Kamke, M. Petrič, and M. Sernek, The influence of viscoelastic thermal com-pression on the chemistry and surface energetics of wood. Colloids Surfaces 329, 82–86 (2008).

178. P. Boruszewski, P. Borysiuk, M. Mamiński, and M. Grześkiewicz, Gluability of thermally modi-fied beech (fagus silvatica l.) and birch (betula pubescens ehrh.) wood. Wood Mater. Sci. Eng. 6, 185–189 (2011).

179. O. Unsal, Z. Candan, and S. Korkut, Wettability and roughness characteristics of modified wood boards using a hot-press. Ind. Crop. Prod. 34, 1455–1457 (2011).


180. M. Petrič, B. Knehtl, A. Krause, H. Militz, M. Pavlič, M. Pétrissans, A. Rapp, M. Tomažič, C. Welzbacher, and P. Gérardin, Wettability of waterborne coatings on chemically and thermally modified pine wood. J. Coat. Technol. Res. 4, 203–206 (2007).

181. J.D. Jennings, A. Zink-Sharp, C.E. Frazier, and F.A. Kamke, Properties of compression-densified wood, Part II: Surface energy. J. Adhesion Sci. Technol. 20, 335–344 (2006).

182. A. Kutnar, L. Rautkari, K. Laine, and M. Hughes, Thermodynamic characteristics of surface densified solid Scots pine wood. Eur. J. Wood Wood Prod. 70, 727–734 (2012).

183. M. Stehr, D.J. Gardner, and M.E. Wålinder, Dynamic wettability of different machined wood surfaces. J. Adhesion 76, 185–200 (2001).

184. V. Landry and P. Blanchet, Surface preparation of wood for application of waterborne coatings. Forest Prod. J. 62, 39–45 (2012).

185. A. Özçifçi and F. Yapici, Effects of machining method and grain orientation on the bonding strength of some wood species. J. Mater. Process. Technol. 202, 353–358 (2008).

186. J. Follrich, O. Vay, S. Veigel, and U. Müller, Bond strength of end-grain joints and its dependence on surface roughness and adhesive spread. J. Wood. Sci. 56, 429–434 (2010).

187. S. Kuljich, J. Cool, and R.E. Hernández, Evaluation of two surfacing methods on black spruce wood in relation to gluing performance. J. Wood. Sci. 59, 185–194 (2013).

188. R.E. Hernández and J. Cool, Evaluation of three surfacing methods on paper birch wood in relation to water-and solvent-borne coating performance. Wood Fiber Sci. 40, 459–469 (2008).

189. J. Cool and R.E. Hernández, Performance of three alternative surfacing processes on black spruce wood and their effects on water-based coating adhesion. Wood Fiber Sci. 43, 365–378 (2011).

190. J. Cool and R.E. Hernández, Effects of peripheral planing on surface characteristics and adhe-sion of a waterborne acrylic coating to black spruce wood. Forest Prod. J. 62, 124–133 (2012).

191. S.-Z. Chow, Infrared spectral characteristics and surface inactivation of wood at high tempera-tures. Wood Sci. Technol. 5, 27–39 (1971).

192. A. Ugovšek, A. Sever Škapin, M. Humar, and M. Sernek, Microscopic analysis of the wood bond line using liquefied wood as adhesive. J. Adhesion Sci. Technol. 27, 1247–1258 (2012).

193. M. Kishino and T. Nakano, Artificial weathering of tropical woods. Part 1: Changes in wettabil-ity. Holzforschung 58, 552 (2004).

194. X. Huang, D. Kocaefe, Y. Kocaefe, Y. Boluk, and A. Pichette, Changes in wettability of heat-treated wood due to artificial weathering. Wood Sci. Technol. 46, 1215–1237 (2012).

195. J.J. Paredes, R. Mills, S.M. Shaler, D.J. Gardner, and A. van Heiningen, Surface characterization of red maple strands after hot water extraction. Wood Fiber Sci. 41, 38–50 (2009).

196. T. Nguyen and W. Johns, The effects of aging and extraction on the surface free energy of Douglas-fir and redwood. Wood Sci. Technol. 13, 29–40 (1979).

197. P. Northcott, The effect of dryer temperatures upon the gluing properties of Douglas-fir veneer. Forest Prod. J. 7, 10–16 (1957).

198. G. Sinclair, R. Evans, and H. Sallans, New methods for sizing fibrous products. Pulp Paper Mag. Can. 62, T23-T27 (1960).

199. E. Roffael and W. Rauch, Extraktstoffe in Eiche und ihr Einfluss auf die Verleimbarkeit mit alkalischen Phenol-formaldehydharzen. Holz Roh Werkst. 32, 182–187 (1974).

200. M. Gindl, G. Sinn, and S.E. Stanzl-Tschegg, The effects of ultraviolet light exposure on the wet-ting properties of wood. J. Adhesion Sci. Technol. 20, 817–828 (2006).


201. K.T. Lu and S.Y. Fan, Effects of ultraviolet irradiation treatment on the surface properties and adhesion of moso bamboo (Phyllostachys pubescens). J. Appl. Polym. Sci. 108, 2037–2044 (2008).

202. G.S. Oporto, D.J. Gardner, G. Bernhardt, and D.J. Neivandt, Forced air plasma treatment (FAPT) of hybrid wood plastic composite (WPC)-fiber reinforced plastic (FRP) surfaces. Composite Interfaces 16, 847–867 (2009).

203. T. Sellers, Jr., Adhesives in the wood industry, in Handbook of Adhesive Technology, A. Pizzi and K.L. Mittal (Eds.), pp. 599–614, Marcel Dekker, New York, NY. (1994).

204. K.L. Johnson, Contact Mechanics, p. 452, Cambridge University Press, Cambridge. (1987).205. B.V. Derjaguin, V.M. Müller, and Y.P. Toporov, Effect of contact deformations on the adhesion

of particles. J. Colloid Interface Sci. 53, 314–326 (1975).206. D.J. Gardner, W.T. Tze, and S.Q. Shi, Adhesive wettability of hydroxymethyl resorcinol (HMR)

treated wood, in Procedings of the Wood Adhesives 2000 Conference, pp. 321–327, Forest Products Society, Madison, WI. (2000).

本文献由“学霸图书馆-文献云下载”收集自网络，仅供学习交流使用。

学霸图书馆（www.xuebalib.com）是一个“整合众多图书馆数据库资源，

提供一站式文献检索和下载服务”的24 小时在线不限IP

图书馆。

图书馆致力于便利、促进学习与科研，提供最强文献下载服务。

图书馆导航：

图书馆首页文献云下载图书馆入口外文数据库大全疑难文献辅助工具

http://www.xuebalib.com/cloud/

http://www.xuebalib.com/

http://www.xuebalib.com/cloud/


http://www.xuebalib.com/vip.html

http://www.xuebalib.com/db.php

http://www.xuebalib.com/zixun/2014-08-15/44.html


adhesion theories in wood adhesive bondingdownload.xuebalib.com/xuebalib.com.12740.pdf · adhesion...

Documents